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Practical Applied 

ELECTRICITY 



A BOOK IN PLAIN ENGLISH 
FOR THE PRACTICAL MAN. 
THEORY, PRACTICAL AP- 
PLICATIONS AND EXAMPLES 



Bv 
DAVID PENN MORETON, B.S., E.E. 

ASSOCIATE PROFESSOR OF ELECTRICAL ENGINEERING AT ARMOUR 
INSTITUTE OF TECHNOLOGY, ASSOCIATE MEMBER OF THE AMERI- 
CAN INSTITUTE OF ELECTRICAL ENGINEERS, MEMBER 
OF THE SOCIETY FOR THE PROMOTION OF 
ENGINEERING EDUCATION, ETC. 



ILLUSTRATED 



Publishers 

The Reilly & Britton Co. 

Chicag-o 



COPYRIGHT, 1911 

By DAVID PENN MORETON 

CHICAGO 






©CI.A295311 



DEDICATED 

TO MY 

Father and Mother 
CHARLES FROWN MORETON 

AND 

SALLIE ^ENN MORETON 

IN 
APPRECIATION OF THEIR UNTIRING 
EFFORTS WHILE I WAS RE- 
CEIVING MY EARLY 
EDUCATION 



I 



PREFACE 



This book is intended primarily for those persons 
who are desirous of obtaining a practical knowledge of 
the subject of Electricity, but are unable to take a com- 
plete course in Electrical Engineering. It is the opinion 
of the author that such persons should have a thorough 
understanding of the fundamental principles of the sub- 
ject, in order that they may easily understand the 
applications in practice. Numerous examples are solved 
throughout the book, which serve to illustrate the prac- 
tical application of certain laws and principles and 
give the reader an opportunity to more readily grasp 
their true significance. 

The text is based, to a certain extent, upon a series 
of lectures given in the evening classes in the Depart- 
ment of Electrical Engineering at Armour Institute of 
Technology. The arrangement is not the one usually 
followed, and to some it may not appear to be logical; 
but it is one the author has found very satisfactory. 

Although the book was not originally intended to be 
used as a text-book, it is, however, especially adapted 
for use in the practical courses given in the various 
High and Manual Training Schools, and at the same 
time gives a substantial groundwork for the more 
advanced college and university courses. 

The author wishes to express his thanks to the 
various manufacturing companies who havfe been very 
kind in supplying material and cuts, and to Professor 
E. H. Freeman, head of the Department of Electrical 
Engineering of Armour Institute of Technology, for a 
number of valuable suggestions. 

DAVID PENN MORETON 

Armour Institute of Technology 
June, 1911 



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CONTENTS 



I Electrical Circuit and Electrical 
Units 

II Calculation of Resistance 

III Series AND Divided Circuits, Measure 

MENT OF Resistance 

IV Primary Batteries 

V Magnetism 

VI Electromagnetism 

VII Electromagnetic Induction, Funda- 
mental Theory of the Dynamo 

VIII Electrical Instruments and Effects 
OF A Current 

IX Direct -Current Generator 

X Direct -Current Motors 

XI ARMATtJRES FOR DiRECT-CuRRENT DyNA 
MOS ..... 

XII Storage Batteries, their Applica 
TioNS AND Management 

XIII Distribution and Operation 

XIV Diseases of Direct-Current Dyna 

MOS ..... 

XV Electric Lighting . . 

XVI Electric Wiring 

XVII Alternating-Current Circuit 

XVIII Alternating-Current Machinery 

XIX Resuscitation .... 

XX Logarithms and Reference Tables 



1 
18 

34 

58 

77 
86 

102 

121 
166 
193 

219 

237 
257 

280 
294 
316 
332 
358 
386 
390 



r 



PRACTICAL APPLIED ELECTRICITY 



CHAPTER I 

THE ELECTRICAL CIRCUIT AND ELECTRICAL UNITS 

1. Electricity. — There are certain phenomena of nature 
that are taking place about us every day which we call elec- 
trical, and to that which produces these phenomena we give 
the name electricity. The exact nature of electricity is not 
known, yet the laws governing its action, under various con- 
ditions, are well understood, just as the laws of gravitation 
are known, although we cannot define the constitution of 
gravity. Electricity is neither a gas nor a liquid; its be- 
havior sometimes is similar to that of a fluid so that it is 
said to flow through a wire. This expression of flowing does 
not really mean there is an actual movement in the wire, 
similar to the flow of water in a pipe, when it possesses elec- 
trical properties, but is simply a convenient expression for 
the phenomena involved. According to the modern electron 
theory of electricity, there is a movement of some kind tak- 
ing place in the circuit, but the real nature of this move- 
ment is not as yet very well understood. A great many electrical 
problems can be easily understood by comparing them to a 
similar hydraulic problem, where the relation of the various 
quantities and the results are apparent under given condi- 
tions, and on account of the similarity of the two, the hy- 
draulic analogy will often be used to illustrate what actu- 
ally lakes place in the electrical circuit. 

2. Electrical Circuit. — The electrical circuit is the path in 
which the electricity moves. The properties of electrical cir- 
cuits and the electrical quantities associated with them make 
an appropriate beginning for the study of the subject of 
electrical engineering, for electrical energy is in almost all 

] 



2 



PRACTICAL APPLIED ELECTRICITY 



practical applications utilized in circuits. These circuits are 
of various forms and extent, and are made for many different 
purposes, but all possess to a greater or less extent the same 
properties and involve the same electrical quantities. Sup- 
pose, for example, a door bell that is operated from two 



h f-^ ^^ Y --. 



Battery 




Push Button 

Fig. 1 



or three dry cells, as shown in Fig. 1. This combination 
forms an electrical circuit, typical of all electrical cir- 
cuits. It contains a source of electrical energy (the bat- 
tery), an energy transforming device (the bell), and the 
necessary connecting materials (wire and push button). It 
is closed upon itself and like the circumference of a circle 
has neither beginning nor end. (A table of symbols com- 
monly used in representing the different pieces of electrical 
apparatus is given in Chapter 20.) 

3. Hydraulic Analogy of the Electrical Circuit. — Before con- 
sidering the relation of the various electrical quantities asso- 
ciated with the electric circuit it would, no doubt, be best 
to make a brief study of a simple hydraulic problem, simi- 
lar to the electrical circuit, and the relation of the various 
quantities involved. It must be clearly understood that the 
similarity between the water and the electrical circuits is in 
regard to action only, and does not in any way imply an 
identity between electricity and water. A simple hydraulic 
circuit is shown in Fig. 2, where (P) is a pump that sup- 
plies water, under a constant pressure, to the pipe (T). The 
water is conducted through the pipe (T) to the tank (K), 
the amount of water flowing through the pipe being regu- 
lated by the valve (V). The pipe (T) is composed of a 
number of different pieces of pipe, differing in area of 



THE ELECTRICAL CIECinT 



8 



cross-section, length, and condition of their inner surface, 
some being rough and some smooth. Pressure gauges (Gi, 
G2, etc.) are placed along the pipes at certain intervals, as 
shown in the figure, to indicate the pressure in the pipe at 
different points. Assume, first, that the valve (V) is closed; 
no water will then be flowing through the pipe and all of 
the various pressure gauges will indicate the same pressure 
in the pipe. Assume, as a second condition, that the valve (V) 
is partly opened; there will be a flow of water in the pipe, 
and the pressure gauges will indicate a different pressure, 
their indications decreasing as you pass along the pipe, 
starting from the end attached to the pump. Opening the 




Fig. 2 

valve still more will, increase the flow of water and also 
the difference in indications of the various gauges. If the 
gauge (Gg) at the end of the pipe indicates zero pressure, 
it is apparent that the total pressure supplied by the pump 
has all been used in forcing the water through the pipe. If, 
on the other hand, the last gauge indicates a certain pres- 
sure, then the total pressure supplied by the pump has not all 
been used and there still remains a certain amount, neg- 
lecting that pressure required to force the water through 
the valve (V), that could be used in driving a water wheel, 
placed in such a position that the water could strike against 
the buckets on the wheel and cause it to rotate. The differ- 



4 PRACTICAL APPLIED ELECTRICITY 

ence in pressure indicated by any two gauges is a meas- 
ure of the pressure required to cause the water to flow be- 
tween the two points where the gauges are connected to the 
pipe. If it were possible to have a pipe that would offer no 
opposition to the passage of the water through it, there would 
be no difference in the indications of the various gauges and 
the water would flow from the end of the pipe under the 
same pressure as that produced by the pump. Anything that 
increases the opposition to the flow of the Avater will increase 
the value of the pressure required to cause a certain quan- 
tity to pass through the section of pipe, between the pres- 
sure gauges, whose indications are being noted. Or, if the 
same pressure is maintained over a given section and its 
opposition to the passage of water is increased, the quantity 
passing in a given time will be decreased; similarly the 
quantity will be increased if the opposition is decreased. 
As an example, suppose a water motor is connected in the 
pipe (T) and it is perfectly free to turn, there will then be a 
very small pressure required to cause a certain quantity to 
flow through the motor in a given time. If, however, the 
water motor is loaded, it will offer a much greater resist- 
ance to the passage of water through it; and, if the pres- 
sure remains constant, the quantity passing in a given time 
will be decreased. The rate of flow can be maintained 
constant when the opposition increases by increasing the 
pressure. The total pressure supplied by the pump will be 
distributed over the various sections of the pipe in propor- 
tion to their opposition. The opposition offered by the pipe 
can be called its resistance and it will depend upon the 
area, length, nature of the inner surface, and any obstruc- 
tion that might be in the pipe, such as sand, gravel, or sieves. 
The quantity of water passing a given point in a pipe in a 
unit of time, usually one second, is called the current. These 
three quantities, pressure, current, and resistance, are related 
to each other in a very simple way, as can be seen from 
the above problem, and this relation, expressed in the form 
of an equation is: 

Pressure 

Current (1) 

Resistance 



THE ELECTEICAL CIKCUIT 5 

It must be remembered that this equation does not hold 
in its strictest sense for the hydraulic problem, but will 
serve to illustrate the relation between the electrical quan- 
tities that are to be discussed in one of the following sections. 

4. Electrical Circuit Compared with Hydraulic Circuit. — 
In the electrical circuit, as indicated in Fig. 3, there is a 
source of electrical pressure, the battery, that corresponds 
to the pump in the hydraulic analogy. The wire by means 
of which the electricity is conducted corresponds in the 
hydraulic analogy to the pipe through which the water flows, 




Fig. 3 

while the electric motor (M), in Fig. 3, corresponds to the 
water motor referred to in the example under section (3). If 
all the electrical pressure is consumed in causing a given 
quantity of electricity to flow through the wire in a given 
time, there will be no pressure available to operate the elec- 
trical motor or any other electrical device, just as there would 
be no pressure available to operate the water motor in the 
hydraulic problem when all the pressure was used in caus- 
ing the water to flow through the pipe. The difference in 
electrical pressure between any two points on the wire is a 
measure of the pressure required to cause a given quantity 
of electricity to pass from one of the points to the other 
in a given time. This difference in pressure will depend 
upon the opposition offered by the circuit between the two 
poii^ts and the quanfity passing between them in a given 
time. With an increase in the opposition offered by the cir- 
cuit there must be an increase in pressure to maintain a 
constant flow. If, on the other hand, the pressure remains 
constant and the opposition to the flow increases or decreases, 
there will be a decrease or increase in the quantity of 



6 PEACTICAL APPLIED ELECTEICITY 

electricity passing a certain point in the circuit in a given 
time. 

5. Electrical Quantities.— There are three electrical quan- 
tities associated with the electrical circuit that correspond 
to those given for the hydraulic analogy. These quantities 
are the current of electricity, the electromotive force which 
causes the current, and the resistance which hinders the free 
flow of the electricity. 

6. Current of Electricity.— The rate at which the move- 
ment, or flow, of electricity takes place is the current— for 
a uniform movement it is the amount of electricity flowing 
per unit of time— usually the second. The unit quantity of 
electricity is the coulomb, and if the rate of flow is such 
that one coulomb flows per second, there is said to be a 
current of one ampere. 

At any instant the current is the same at all parts of a 
circuit having no branches, and it is the same for all points 
in each branch, where there are branches. A mistake is 
often made in thinking that the current is used up— that 
there is practically no current returning to the source of 
the electrical energy. Such a condition is not true. The 
current in the return conductor to the battery or dynamo 
is exactly the same as the current in the conductor carry- 
ing the electricity from the battery or dynamo. The cur- 
rent that leaves a motor, lamp, or bell is no different in 
value from that which enters them. It is the energy of the 
electricity that is used up, just as it is the energy possessed 
by the water that is imparted to the water wheel and causes 
it to rotate without the water itself being consumed. The 
symbol used in representing the current is the letter (I). 

7. Resistance.— Resistance is that property possessed by 
substances which opposes the free flow of electricity, but 
does in no way tend to cause a flow in the opposite direction 
to that in which the electricity actually moves. 

All substances resist the movement of electricity through 
them, but the resistance offered by some is very much greater 
than that offered by others, and as a result all materials 
may be grouped under one of two heads, namely, conductors 
and insulators. 

Conductors are those substances offering relatively low 
resistance. 



THE ELECTRICAL CIRCUIT 7 

Insulators are thos'e substances offering a high resistance as 
compared to conductors. 

There is no such thing as a perfect conductor or a perfect 
insulator, but conductors and insulators are merely relative 
terms and define the degree to which a substance conducts. 
Metals have by far the least resistance of any substances 
and are used in electrical circuits where a minimum resist- 
ance is desired. The most common insulators in use are 
porcelain, glass, mica, stoneware, slate, marble, rubber, oils, 
paraffin, shellac, paper, silk, cotton, etc. 

The unit in which resistance is measured is the ohm, and 
it is represented by the symbol (R). 

8. Electromotive Force. — As every part of the circuit offers 
resistance to the fiow of electricity, there must be some 
force, or pressure, to overcome this resistance and maintain 
the current. This is the electromotive force or electricity 
moving force., Electromotive force, or e.m..f., as it is abbre- 
viated, can be generated in a number of different ways; 
perhaps the two most common are the chemical production 
in a primary cell and the mechanical generation by the proc- 
ess of electro-magnetic induction in a generator. Electro- 
motive force does not create electricity, but simply imparts 
energy to it, as a mechanical force may produce motion in 
a body and as a result convey energy to the body. An elec- 
tromotive force may exist without producing a current, just 
as a mechanical force may exist without producing motion. 
The unit in which electromotive force is measured is the 
volt. 

9. Electromotive Force and Potential Difference. — The 
electromotive force in any circuit is the total generated 
electrical pressure acting in the circuit, while the potential 
difference is the difference in electrical pressure between 
any two points in the circuit. The electricity in its passage 
around the circuit loses some of its energy ; hence, between 
any two points in the circuit there will be a difference in 
energy or potential possessed by the electricity. In over- 
coming the resistance of the wire between the points (A) and 
(B), Fig. 4, the electricity will lose some of its energy and 
will, therefore, have less potential at (B) than at (A), or, in 
other words, there is a difference in potential between (A) 
and (B). This difference in potential in the electrical circuit 



8 PEACTICAL APPLIED ELECTRICITY 

is analogous to the difference in pressure between two points 
on the pipe in the hydraulic circuit. This potential differ- 
ence, or p.d., as it is abbreviated, is due to the current through 
the resistance, for when the current stops, the difference in 
potential will no longer exist. The potential difference is 
measured in the same unit as the electromotive force, the 
volt. The sum of all the potential differences in any elec- 
trical circuit is numerically equal to the effective e.m.f. act- 
ing in the circuit. If two points can then be located on a 
circuit so that all the p.d.'s around the circuit will be be- 
tween them, the potential difference between the points and 
the e.m.f. acting in the circuit will be numerically equal. 





Generator 



Fig. 4 

10. Ohm's Law. — Current, electromotive force, and resist- 
ance are always present in an active circuit and there is a 
simple, but very important, relation connecting them. This 
relation was first discovered by Dr. G. S. Ohm in 1827 and 
has, as a result, been called Ohm's Law. Dr. Ohm discovered 
by experiment that the difference of potential, represented 
by the symbol (E), between any two points on a conductor, 
is, all other conditions remaining constant, strictly propor- 
tional to the current (I), or 

E = Constant XI (2) 

By measuring (E) in volts and (I) in amperes, the constant in 
the above equation is numerically equal to the resistance, be- 
tween the points, in ohms. The expression can then be re- 
duced to the form 

Volts = Resistance X Amperes 

E = RI (3) 



THE ELECTEICAL CIRCUIT 9 

or, as it is usually written, 

Amperes = Volts -f- Ohms 
E 

I = (4) 

R 

and as a third form. 

Ohms = Volts -^ Amperes 

R = (5) 

I 

The above relations hold true for all or any part of a cir- 
cuit composed of metals and electrolytes, but it does not 
seem to be true for gases, under certain conditions, nor for 
insulators. 

Example. — The total resistance of a certain circuit is 55 
ohms. What current will a pressure of 110 volts produce in 
the circuit? 

Solution. — The value of the current is given in terms of 
the pressure and the resistance in equation (4) and by a 
direct substitution in this equation we have 

110 

I = = 2 

55 

Ans. 2 amperes. 

Example. — A current of 10 amperes is produced in a circuit 
whose resistance is 12% ohms. What pressure is required? 

Solution.— The electrical pressure is given in terms of the 
resistance and the current in equation (3) and by a direct 
substitution in this equation we have 

E = 121/2 X 10 = 125 

Ans. 125 volts. 
11. Coulomb, Ampere, Ohm, and Volt. — The International 
Electrical Congress that met in Chicago in 1893 officially 
passed upon the following values for the coulomb, ampere, 
ohm, and volt, and they are known as the practical units. 

(a) In defining the coulomb advantage is taken of the 
fact that electricity in passing through a solution of silver 
nitrate deposits silver on one of the conductors immersed 



10 PRACTICAL APPLIED ELECTEICITY 

in the solution and the amount deposited is proportional to 
the quantity of electricity that passed through the solution. 
(See chapter on "Instruments.") The coulomb will deposit, 
under standard conditions, .001 118 of a gram of silver. 

(b) Since the current is the rate of flow of electricity, 
then the ampere is the unit rate of flow, or one coulomb 
per second, and its value is, therefore, that current which 
will deposit silver at the rate of .001 118 of a gram per sec- 
ond. When smaller currents are to be measured, the milli- 
ampere, or one-thousandth of an ampere, and the micro- 
ampere, or one-millionth of an ampere, are frequently used. 

(c) The International ohm, as nearly as known, is the 
resistance of a uniform column of pure mercury 106.3 centi- 
meters long and 14.4521 grams in mass, at the temperature 
of melting ice. This makes the cross-section one square 
millimeter. The microhm is one-millionth of an ohm and 
the megohm is one million ohms. These units are very fre- 
quently used in the measurement of very small or very large 
resistances. 

(d) The volt, the unit of electromotive force or poten- 
tial difference, is that e.m.f. or p.d. which, when applied to 
a conductor having a resistance of one ohm, will produce in 
it a current of one ampere. One volt equals the 1^^91434 P^^^ 
of the e.m.f. of a Clark Standard Cell at 15 degrees centi- 
grade. A smaller unit, called the millivolt, or one-thousandth 
of a volt, is often used in measuring small e.m.f.'s and p.d.'s. 
Another unit, called the kilovolt, has a value of one thousand 
volts. 

PROBLEMS ON OHM'S LAW 

(1) The difference in potential between the terminals of 
a generator is 110 volts. If the generator is causing a cur- 
rent of 12 amperes in the circuit to which it is connected, 
w^hat is the resistance of the circuit? 

Ans. 9.17 ohms. 

(2) What e.m.f. must a dynamo generate to supply an 
electroplating current of 20 amperes through a circuit whose 
total resistance is .2 ohms? Ans. 4 volts. 

(3) An electric-car heater is supplied with a p.d. of 500 



THE ELECTRICAL CIRCUIT U 

volts from the trolley. What must its resistance be in or- 
der that the current may not exceed 8 amperes? 

Ans. 62.5 ohms. 

(4) The difference in potential between the terminals of 
an incandescent lamp is 110 volts when there is a direct 
current of .5 ampere through the lamp. What is the re- 
sistance of the lamp? Ans. 220 ohms. 

(5) The field circuit of a dynamo takes a current of 1.5 
amperes from a 110-volt circuit. What is the resistance of 
the field winding? Ans. 73^3 ohms. 

(6) A certain bell requires a p.d. at its terminals of 8 
volts to operate it. The bell has a resistance of 50 ohms. 
What current is required to operate the bell? 

Ans. .16 ampere. 

(7) An e.m.f. of 5 volts is connected to a line and bell 
having a combined resistance of 12 V2 ohms. What current 
is there through the bell? Ans. .4 ampere. 

(8) A lamp has a hot resistance of 55 ohms and requires 
a current of 1 ampere to cause it to light up to full candle- 
power. What p.d. must exist between the terminals of the 
lamp when it burns at full candle-power? 

Ans. 55 volts. 

12. Electrical Force. — Force is defined as that which 
tends to produce motion, or a change of motion; thus a force 
must always be applied to a body to cause it to move, and 
a force must be again applied to cause the body to come 
to rest. It must be remembered that a force does not al- 
ways produce motion, but only tends to produce it, as when 
you push on a brick wall you apply muscular force, but there 
is no motion. There are a number of different kinds of 
force: Gravitational force, as a result of which all bodies fall 
from a higher to a lower level: mechanical force, which is 
produced by the expansion of the steam in the engine cylin- 
der, and may be used in driving a generator; and electrical 
force is that force which produces or tends to produce a 
movement of electricity. It is commonly produced in the pri- 
mary battery or by an electrical generator. 



12 PRACTICAL APPLIED ELECTRICITY 



13. Electrical Work or Energy. — When a force overcomes 
a certain resistance, work is done; or work is the result of 
a force acting through a certain distance. Force may exist 
without any work being done. Thus, a generator may be op- 
erating and generating an electrical force, but it is not 
sufficient to overcome the resistance between the terminals 
of the machine, therefore, no current is produced and the 
generator is not doing any work. If, however, a conductor 
be connected to the terminals of the generator, a current 
will be produced and as a result the generator will do work. 
The electrical work done by the dynamo will show itself 
as heat and the wire will become heated as a result. En- 
ergy is the ability to do work. Electrical energy, then, is 
the capacity or ability to do electrical work. Energy is 
measured in the same unit as work and it is numerically 
equal to the work done. Thus the energy possessed by a 
certain quantity of electricity, with respect to its energy at 
some- other electrical level, is equal to the work done on or 
by the quantity in moving from the first to the second 
electrical level. When a quantity moves from a higher to 
a lower level, it gives up energy or does work; when it Is 
moved from a lower to a higher electrical level, work is 
performed and the energy of the quantity of electricity is 
increased. 

14. — The Joule. — The joule, the unit of electrical work 
or energy, is the amount of work performed in raising one 
coulomb of electricity through a difference in electrical 
pressure of one volt. Work is independent of the time. 
In other words, it will require the same amount of work 
to raise a certain weight a given height in one hour as 
would be required to raise it the same height in one minute. 
The name for the mechanical unit of work Is a compound 
word in which one of the parts is a force unit and the other 
part IS a distance unit. The unit commonly used is the foot- 
pound, the pound being the unit of force and the foot the unit 
of distance. The electrical work performed in an electrical 
circuit is equal to 

Joules •== Volts X Amperes X Time (in seconds) (6) 

Amperes times time is equal to the quantity of electricity 
moved, so that the right-hand portion of the above equation 



THE ELECTEICAL CIECUIT 13 

is the product of a certain quantity and the difference in 
electrical pressure through which it is moved. 

15. Electrical Power. — Power is the rate of doing work, 
or the rate at which energy is expended. The unit of elec- 
trical power is a unit of electrical work performed in a unit 
of time, or a joule per second, and it is called the watt, rep- 
resented by the symbol (W). 

Electrical Work 

Electrical Power= (7) 

Time 

Substituting the value of the electrical work given in equa- 
tion (6) in equation (7), we have 

Volts X Amperes X Time 

Electrical Power = 

Time 

== Volts X Amperes, or (8) 

W =EI (9) 

One watt, therefore, equals one volt multiplied by one am- 
pere, or any product of volts and amperes whose result is 
unity. Equation (9) may be written 

W 

I = (10) 

or E 

W 

E= (11) 

I 

Example. — ^A 220-volt generator supplies 20 amperes, to a 
motor. How many watts does the motor consume? 
Solution. — Substituting directly in equation (9) we have 

W = 220 X 20 = 4400 

Ans. 4400 watts. 

Example. — An arc lamp requires 880 watts from a 110- volt 
ctircuit to operate it. What current does the lamp take? 
Solution. — Substituting in equation (10) we have 



14 PKACTICAL APPLIED ELECTRICITY 

880 

I = = 8 

110 

Ans. 8 amperes. 

16. Mechanical Horse-Power. — If a body weighing 33000 
pounds be raised one foot in one minute, there is a rate of 
working, or expenditure of energy, equivalent to one horse- 
power (abbreviated h.p.). The horse-power any machine 
is developing is equal to the .foot-pounds of work done per 
minute divided by 33 000, or the foot-pounds of work done \ 
per second divided by 550. j 

17. Relation between the Watt and the Foot-Pound. — Dr. ; 
Joule discovered experimentally that 



or 



One watt = .7375 foot-pound per second (12) 

One foot-pound per second = 1.356 watts (13) 



18. Electrical Horse-Power. — Since the mechanical horse- 
power is 550 footpounds per second, an equivalent rate of 
doing work would be 

550 
■■ — = 746 watts = 1 electrical horse-power (14) 



.7375 
Then to change from mechanical horse-power to the elec- 
trical units, multiply by 746, or 

Watts = h.p. X 746 (15) 

To determine the horse-power a generator is developing, de- 
termine its output in watts and divide by 746, or 

E X I watts 

hp.= = (16) 

746 746 

19. Kilowatt. — The kilowatt (abbreviated k.w.) is a larger 
unit of electrical power than the horse-power. It is equal 
to 1000 watts or about 1% horse-power. 

20. Larger Units of Energy. — The joule is a very small 
unit of electrical energy or work, and as a result larger 




THE ELECTEICAL CIECUIT 15 

nits are usually used in practice. The watt-hour is one 
watt expended for one hour. The kilowatt-hour is equal to 
1000 watts expended for one hour. The watt-hour is equiva- 
lent to 3600 watt-seconds or 60 watt-minutes. 

Watt-hours = watts X hours ^ (17) 

Kilowatt-hours = k.w. X hours (18) 

Horse-power hour =^ h.p. X hours (19) 

The dials of the integrating wattmeters used by central 
station companies usually record the energy supplied to the 
consumer for lighting or power purposes in watt-hours or 
kilowatt-hours. 

Example. — Ten Incandescent lamps take a current of 2^-2 
amperes from a 220-volt circuit. "What will it cost to operate 
these lamps for fi\e hours if the power company charges 
14 cents per kilowatt-hour? a ,;jvl - -mj^^ < : 

Solution. — Substituting in equation (9), we,)a^^l dft^.r^i§i|e 
the power taken by the lamps in watts: ; ixai,yi^>t c^^'i 

Power = 220 X 2V2 =- 550 watts ' " ;^^^^^^ ' ' 

Now by substituting in equation (17), we can determine the 
energy consumed in watt-hours 

Energy == 550 X 5 = 2750 watt-hours 

One kilowatt-hour is equivalent to 1000 watt-hours, hence 2750 
watt-hours are equal to 2.75 kilowatt-hours. The total cost 
then would be 

2.75 X 14 = 381/2 

Ans. 381/4 cents. 

21. Units. — Units are of two kinds — fundamental and de- 
rived. The fundamental units are the ones from which all 
others are derived and in terms of which all physical meas- 
urements may be made. They are fundamental in the sense 
that no onfe is derived from the others. The derived units 
are all those that are dependent for their definition upon the 
fundamental ones. 

The three fundam.ental units in use are those of length, 
mass, and time. Of these the only unfamiliar one may be 



16 



PKACTICAL APPLIED ELECTEICITY 




that of mass, which means the quantity of matter ir 
body. From these three fundamental units are derived all 
others in use, such, for example, as those of force, work, 
and energy already mentioned, and all of the electrical units. 

22. Systems of Units. — Unfortunately there are two sys- 
tems of units in use in this country. One is the ordinary 
commercial, or English System, and the other the universal 
scientific, or Metric Systsm. In each of these systems we 
have the three fundamental units and also a set of derived 
units. It is quite essential that you know something of 
the metric system, since all the electrical units are based 
upon it, and a great many dimensions will be given in unite 
that belong to this system. Table No. I will show the most 
common units in the two systems. 

The English System is sometimes spoken of as the foot- 
pound-second (f.p.s.) system, because of the three funda- 
mental units upon which it is based. Similarly the Metric 
System is frequently spoken of as the centimeter-gram-sec- 
ond (c.g.s.) system. 

The relation between the units of the two systems is given 
in Table A, Chapter 20. 



TABLE NO. I 



RELATION OF UNITS IN ENGLISH AND METRIC SYSTEMS 





'^ 


UNITS 




SYSTEM 


LENGTH 


MASS 


TIME 


English or 
(F, P. S.) 


Yard (yd.) 
Foot (ft.) 


Pound (lb.) 
Ounce (oz.) 


Second (sec.) 


Metric or 
(C. G. S.) 


Meter (m,) 
Centimeter (cm.) 


Kilogram (kg.) 
Gram (g.) 


Second (sec.) 



PROBLEMS ON POWER AND ENERGY CALCULATIONS 



(1) If 4000 watts are expended in a circuit, how many 
horse-power are being developed? 

Ans. 5.36+ horse-power. 



THE ELECTRICAL CIRCUIT 17 

(2) If 20 horse-power of mechanical energy were converted 
into electrical energy, how many watts would be developed? 

Ans. 14 920 watts. 

(3) One hundred and twenty-five horse-power expended 
continuously for one hour will produce how many kilowatt- 
hours? Ans. 93.25 k.w.-hours. 

(4) How many watts are expended in a 110-volt, 16- 
candle-power lamp that requires a current of .5 ampere? 

Ans. 55 watts. 

(5) How many horse-power will be absorbed by a circuit 
of series arc lamps taking 9.6 amperes, the line pressure 
being 2000 volts ' Ans. 25.73 + horse-pow.er. 

(6) A motor takes a current of 5 amperes from a 110- 
volt circuit. What will it cost to operate this motor for 10 
hours, if the centraL station company charges 12 cents per 
kilowatt-hour? Ans. 66 cents. 



CHAPTER II 
CALCULATION OF RESISTANCE 



n 



23. Resistance. — Resistance has been defined as a prop- 
erty of materials which opposes the free flow of electricity- 
through them. The value of the resistance of any conductor 
in ohms will, however, depend upon the dimensions, tempera- j 
ture, and the kind of material of which it is composed. I 

24. Conductance. — The inverse of resistance is known as ; 
the conductance of a conductor. That is, if a conductor ] 
has a resistance of (R) ohms, its conductance is equal to 5 
(l-^R). The unit in which the conductance is measured is ' 
the mho, and it is the conductance offered by a column of pure 
mercury 106.3 cm. long and 14.4521 grams in mass at the 
temperature of melting ice. This unit is little used in prac- 
tice except in the calculation of the resistance of^a divided 
circuit, which will be taken up later. 

25. Resistance Varies Directly as the Length of a Con- 
ductor.— The resistance of a conductor is directly propor- 
tional to its length, the temperature and cross-section of the 
conductor remaining constant. That is, the resistance in- 
creases at the same rate the length of the conductor in- J 
creases, just as the resistance of a pipe to the flow of wa- ) 
ter increases as the length of the pipe is increased, all J 
other conditions remaining constant. Hence, if the length 
of a conductor is increased to four times its original value, 
the resistance will be four times as much; or if the length 
is divided into four equal parts, the resistance of each part 
will be one-fourth of the total resistance. 

Example. — The resistance of a piece of wire fifteen feet 
long is 5 ohms. What is the resistance of 1000 feet of the 
same kind of wire? 

18 



CALCULATION OF EESISTANCE 19 

Solution. — Since the resistance of a conductor is directly 
proportional to its length, we can determine the resistance 
of one foot of the wire, knowing the resistance of five feet, 
by dividing 5 by 15: 

5 ^ 15 - i/a 
The resistance of one foot, then, is % ohm, and the resist- 
ance of 1000 feet would be 1000 times as much, or 
1000 X Vs = 3331/3 

Ans. 333 1^ ohms. 
Example. — The resistance of a certain conductor thirty 
feet long is 30 ohms. What is the resistance of three inches 
of the conductor? 

Solution. — The length of the conductor in inches is equal 
to the number of inches per foot (12) multiplied by the num- 
ber of* feet: 

12 X 30 = 360 
Since the length is 360 inches and the resistance is 30 ohms, 
the resistance per inch can be obtained by dividing 30 by 
360: 

30 -J- 360 = 1^2 

The resistance of one inch is then 1^2 ohm and the resist-, 
ance of three inches will be three times one-twelfth: 

3 X ii2 = %2 = % 

Ans. ^ ohm. 

26. Resistance Varies Inversely as the Cross-Section of a 
Conductor. — The resistance of a conductor is inversely pro- 
portional to the cross-section, the temperature and length 
of the conductor remaining .constant. That is, the resist- 
ance decreases at the same rate the area of the cross-sec- 
tion increases, all other quantities remaining constant. 
Hence, if the area of a conductor is reduced to one-fourth 
its original value, the resistance will be four times as much, 
or if its area is increased, to four times its previous value 
the resistance will be decreased to one-fourth its original 
value. 

Example. — A conductor ^^^qq square inch in area has a 
resistance of .075 ohm per foot. What is the resistance 
of a conductor of the same material i/^5 of a square inch 
in area and one foot long? 



20 



PRACTICAL APPLIED ELECTRICITY 



Solution. — Since the resistance varies inversely as the re- 
lation between the areas, and the relation between the areas 
in this case is 

the resistance of the conductor of larger cross-section will 
be 1/4 the resistance of the conductor of smaller cross- 
section. 

% of .075 = .018 75 

Ans. .018 75 ohm. 
27, Area of Circular Conductors. — Most all conductors 
have a circular cross-section; and it is necessary to know 
how to calculate their areas in terms of their diameters or 
radii in order to determine the relation between their re- 
sistances. The area oi any circle is obtained by multiplying 
the radius by itself and this product by a constant called 
Pi. The value of this constant is 3.1416, and it is the num- 
ber of times the diameter must be used in order to reach 
around the circle. It is represented by the Greek symbol (tt). 
Hence, the area of any circle can be obtained by using the 
equation 



Area = r2 x tt 
Area =^ radius X radius X pi 



(20) 



Since the diameter of a circle is equal to twice the radius, 
this equation may be rewritten in terms of the diameter, 

<d> 



Area 



-© 



X TT 



Area = — x d- 

4 
Area = .7854 d^ 



(21) 



From the above equation it is seen that the area of one 
circle bears the same relation to the area of another as exists 
between their respective diameters squared. That is, if two 
circles have diameters 3 and 5, the relation between their 
areas will be the relation between (3)- and (5)-, or 9 and 25. 
The circle of smaller diameter will have %r, the area of 
the circle of larger diameter. 

Example. — The resistance of a wire .1 inch in diameter and 



CALCULATION OF RESISTANCE 21 

10 feet long is 10 ohms. What is the resistance of a wire 
of the same material and same length .3 inch in diameter? 

Solution. — The ratio of the areas of the two wires will be 
the ratio between ,V and .3^ which is .01 and .09. The larger 
wire will have 9 times the area of the smaller wire and since 
the resistance varies inversely as the area it will have one- 
ninth the resistance. 

1/9 of 10 = !%=!% 

Ans. 1% ohms. 

28. Resistance Clianges with Temperature. — A change in 
the temperature of all substances will cause a change in their 
resistance. In the majority of cases an increase in tempera- 
ture means an increase in resistance. Carbon is the best 
example of substances whose resistance decreases with an 
increase in temperature. The resistance of the carbon fila- 
ment of an incandescent lamp is about twice as great when 
the lamp is cold as when it is lighted. The change in resist- 
ance due to a change in temperature is quite different for 
different materials, and it is also slightly different for the 
same materials at different temperatures. Alloys, such as 
manganin, have a very small chance in resistance due to a 
change in temperature, and standard resistances are, as a 
rule, made from such alloys, as it is desired to have their 
resistance remain as near constant as possible. 

29. Temperature Coefficient. — The temperature coefficient 
of a material is defined as the change in resistance per ohm 
due to a change in temperature of one degree. The resistance 
at 0° centigrade or 32° Fahrenheit is usually taken as the 
standard resistance, which we will call Ro. Now, let the 
resistance at some higher temperature (t) be measured and 
call it Rt. Then Ro — Rt is the change in resistance for a 

Rt — Ro 

change in temperature of (t) degrees, and — is the 

Ro 
change in resistance per ohm for the given change in tem- 
perature (t). The change in resistance for each ohm due to 
a change in temperature of one degree is 

Rt — Ro 

= a (22) 

Rot 



22 PRACTICAL APPLIED ELECTRICITY 

in which (a) is the temperature coeflacient. When the re- 
sistance increases with an increase in temperature, as in 
the case of metals, then Rt is greater than Ro and (a) is 
positive; if Rt decreases with a rise of temperature, as in 
the case of carbon, then (a) is negative. The above ex- 
pression for the temperature coefficient can be changed to 
the form 

Rt = Ro (1 + at) (23) 

This equation gives the value of the resistance (Rt) at any 
temperature (t) in terms of the resistance (Ro) the tempera- 
ture coefficient (a), and the change in temperature (t). 

30. Values of Temperature Coefficients. — Table No. II gives 
the value of the temperature coefficient for a number of 
materials when the temperature change is expressed in centi- 
grade (ac) and Fahrenheit (ar) degrees. 



TABLE NO. II 
TEMPERATURE COEFFICIENTS 

Material ^ ac a>t 

Aluminum 0.004 35 0.002 417 

Carbon 0.000 3 0.000 17 

Copper 0.004 20 0.002 33 

Iron 0.004 53 0.002 52 

Lead (pure) 0.004 11 0.002 2 

Mercury 0.000 88 0.000 49 

Nickel 0.006 22 0.003 45 

Platinum 0.002 47 0.001 37 

Silver 0.003 77 0.002 1 

Tin 0.004 2 0.002 3 

Zinc (pure) 0.004 0.002 2 

The relation between the two coefficients as given in the 
table is the same as that between the centigrade and the 
Fahrenheit degree; that is at = % ac. 

Table B, Chapter 20, gives the relation between the cen- 
tigrade and Fahrenheit thermometer scales. 

The above values of the temperature coefficients are based 
upon a change in resistance, due to a change in temperature 



CALCULATION OF RESISTANCE 



33 



from 0° centigrade or 32° Fahrenheit. If the original tem- 
perature of the conductor is not 0° centigrade or 32° Fahren- 
heit, the value of the temperature coefficient will not be the 
same as that given in Table No. XL There will be a different 
value obtained for (a) for each different initial temperature. 
Table No. Ill gives the change in the value of (a) for copper 
for initial temperatures from 0° to 50° centigrade. 



TABLE NO. Ill 

VALUES OP THE TEMPERATURE COEFFICIENT OF COPPER AT 
DIFFERENT INITIAL TEMPERATURES CENTIGRADE 



Initial 
emperature 
deg. Cent. 


Temp, coefficient 

per 

deg. Cent. 


Initial 
temperature 
deg. Cent. 


Temp, coefficient 

per 

deg. Cent. 





.004 200 


26 


.003 786 


1 


.004 182 


27 


.003 772 


2 


.004165 


28 


.003 758 


3 


.004 148 


29 


.003 744 


4 


.004 131 


30 


.003 730 


5 


.004 114 


31 


.003 716 


6 


.004 097 


32 


.003 702 


7 


.004 080 ' 


33 


.003 689 


8 


.004 063 


34 


.003 675 


9 


.004 047 


35 


.003 662 


10 


.004 031 


36 


.003 648 


11 


.004 015 


37 


.003 635 


12 


.003 999 


38 


.003 622 


13 


.003 983 


39 


.003 609 


14 


.003 967 


40 


.003 596 


15 


.003 951 


41 


.003 583 


16 


.003 936 


42 


.003 570 


17 


.003 920 


43 


.003 557 


18 


.003 905 


44 


.003 545 


19 


.003 890 


45 


.003 532 


20 


.003 875 


46 


.003 520 


21 


.003 860 


47 


.003 508 


22 


MS 845 


48 


.003 495 


23 


.003 830 


49 


.003 483 


24 


.003 815 


50 


.003 471 


25 


.003 801 







24 PRACTICAL APPLIED ELECTEICITY 

31. Calculation of Resistance Due to Change in Tempera- 
ture. — When the resistance of a conductor at a given tem- 
perature is known, which we will assume is 0° centigrade, 
its resistance at some other temperature may be calculated 
by the use of the temperature coefficient. If it is desired 
to calculate the resistance of a conductor at a higher tem- 
perature than that at which it was measured, proceed as 
follows: Multiply the temperature coefficient by the change 
in temperature and the product will be the change in 
resistance of each ohm for the given change in tempera- 
ture; multiplying this product by the original resistance in 
ohms gives the total change in resistance of the conductor. 
This increase in resistance must now be added to the original 
value and the result is the resistance at the second 
temperature. 

The method just given for calculating the resistance with 
a change in temperature can be expressed by the following 
equation : 

Rt = Ro + Ro at (24) 

or, it is usually written 

Rt^Ro (1 + at) (25) 

In the above equation (Rt) is the resistance at the second 
temperature, (Ro) the resistance at the first temperature, 
(0° centigrade), (a) the temperature coefficient, and (t) the 
number of degrees above or below ireezing (0^ centigrade or 
32° Fahrenheit). 

The value of (a) to use in the equation will depend upon 
whether the change in temperature (t) is to be measured 
on the centigrade or on the Fahrenheit scale. 

By an inspection of equation (25) it is apparent that if the 
resistance of a conductor at a higher temperature is known, 
the resistance at a lower temperature can be calculated from 
the equation 

Rt 

Ro=- (26) 

1 -f at 

When the resistance of a conductor at two different tem- 
peratures is known, the change in temperature can be calcu- 
lated from the equation 



CALCULATION OF RESISTANCE 25 

lit — Ro 

t = (27) 

Roa 

Practical use is made of this equation in what is known 
as the resistance thermometer, where the change in resist- 
ance of a coil of wire is measured and the change in tem- 
perature calculated. (Ro) is then the resistance of the coil 
at a known temperature (To), and (Rt) is the resistance of 
the coil at some other temperature (TJ. (TJ is then 
equal to 

T, =-To + t (28) 

If there is an increase in resistance, (t) is positive and if 
there is a decrease, (t) is negative. Hence (TJ will be 
greater or Jess than (To) depending upon the sign of (t). 

Example. — The resistance of a certain copper conductor 
IS 15 ohms at 0° centigrade. What is its resistance at 50° 
centigrade? 

Solution. — The temperature coefficient for copper, when the 
temperature is measured on the centigrade scale, is, from 
Table No. II, found to be .0042. Substituting the values for 
(Ro), (a), and (t)" in equation (25), we have 

Rt =^ 15 (1 + .0042 X 50) = 18.15 

Ans. 18.15 ohms. 

Example. — The resistance of a silver wire is 25 ohms at 
45° Fahrenheit. What resistance will it have at 32° Fah- 
renheit? 

Solution. — The temperature coefficient for silver is found 
from Table No. II to be .0021. Substituting the values for (a), 
(t), which is (45 — 32 =:^ 13), and (Rt) in equation (26), we have 

25 25 

Ro = = == 24.33 

1 -\- 13 X .0021 1.0273 

Ans. 24.33 ohms. 

Example. — A certain platinum coil that is used as a re- 
sistance thermometer has a resistance of 100 ohms at 0° 
centigrade. What is the temperature of the coil when its 
resistance is 115 ohms? 

Solution. — Tl;e temperature coefficient for platinum is found 



26 PRACTICAL APPLIED ELECTKICITY 



1 



from Table No. Ill to be .00247. Substituting the values of the 
resistance and (a) in equation (27), we have 



115 — 100 15 
= = 60.73— 



100 X .002 47 .247 



I 



Since the original temperature (To) of the coil or the one 
at which (Rq) was determined was 0° centigrade, the final 
temperature (TJ can be determined by substituting the values 
of (To) and (t) in equation (28), which gives 

T, = + 60.73 =: 60.73— 

Ans. 60.73 — degrees centigrade 

All of the above calculations are based on an initial tem- 
perature of 0° centigrade or 32° Fahrenheit. It is often the 
case that the resistance of a conductor is known at some 
temperature other than freezing, and it is desired to know 
its resistance at some other temperature. Thus, the resist- 
ance of a certain coil of wire may be known at 20° centi- 
grade and it is desired to know its resistance at 60° centi- 
grade. In this case the resistance of the coil at 0° centi- 
grade should be determined first, by the use of equation 
(26), and the value of (Ro), thus determined, together with 
the value of (a) and (t) substituted in equation (25), which 
will give the value of the resistance at the second tempera- 
ture. The above operations can be combined, which will 
give the equation 

Rt 

Rut, = (1 + ati) (29) 

(1 + at) 

In the above equation (tj is the final temperature and (t) 
is the initial temperature, above or below freezing; (Rt) 
the original resistance; (Rt^tJ the final resistance; and (a) 
the temperature coefficient. If the temperatures are meas- 
ured on the Fahrenheit scale, the values of (t) and (U) 
should be measured above or below 32° depending upon 
whether the temperature be above or below freezing. 

Example. — A coil of platinum wire has a resistance of 100 



CALCULATION OF RESISTANCE 27 

ohms at 45° Fahrenheit. What will the resistance of this 
coil be at 75° Fahrenheit? 

Solution. — This problem can be solved by using equation 
(29). The value of (Rt) to substitute in the equation is 
100; the value of (t) is (45 — 32), or 13; the value of {tj is 
(75 — 32), or 43; and the value of (a) is .00137. Substitut- 
ing the above values in the equation gives the value of 
(Rt+ti), which is the value of the resistance of the coil at 
(ti) degrees above freezing, or in this case 75° Fahrenheit: 
100 (1 + .001 37 X 43) 

Rt+ti = ■ = 104.03 + 

(1 + .001 37 X 13) 

Ans. 104.03+ ohms. ~ 
The error introduced, if the calculation be made without 
reducing the resistance to zero, is not very large, depending 
upon the initial temperature, and in the majority of ordinary 
cases equation (25) can be used. When this equation is used 
(Ro) represents the initial resistance, which should be writ- 
ten (Rt) ; (Rt) represents the final resistance, which should 
be written (Rt+tJ ; and (t) represents the change in tempera- 
ture, which is equal to (tj — t). Equation (25) may then be 
rewritten as follows: 

Rt^ti = Rt [1 + a (ti — t) ] (30) 

Using this equation in calculating the resistance in the last 
problem gives 

Rt^t, = 100 [1 + .001 37 (75 — 43)] 
= 100 (1 + .0411) 
= 104.11 

Ans. 104.11 ohms. 

Equation (29) must then be used when an accurate determi- 
nation of the resistance is desired. Equation (29) can be 
rewritten so that the value of (tj) can be calculated when 
the proper substitution is made: 

[Rt^ti(l + at)]— Rt 

t, = ' (31) 

aRt 

32. Relation of Resistance to Physical Dimensions. — From 
section (26) we know the resistance varies directly as the 
length, and from section (27) inversely as the area of the 



r 



28 PEACTICAL APPLIED ELECTKiClTi 

cross-section. These two relations may be combined with a 
constant, giving the following equation: 

I 
R = K— (32) 

A 
The above equation expresses three facts: 

(a) The resistance (R) varies directly as the length (0 of 
the conductor. 

(b) The resistance varies inversely as the cross-sectional 
area (A) of the conductor. 

(c) The resistance depends upon the material of which 
the conductor is composed. The quality of any material as 
a conductor is expressed by the letter (K), which has a 
definite value for every substance. 

33. Meaning of K. — The physical meaning of (K) in equa- 
tion (32) may be readily determined by making [1) and 
(A) equal to unity. Then length (Z) may be measured in any 
unit of length and the area (A) may be measured in any 
unit of area. (K), then, is the resistance of a conductor 
of unit length and unit cross-section, and there will be as 
many values of (K) as there are units in which we may 
measure (I) and (A). There are two values of (K) that are 
in common use and these only will be considered. They are 
the specific resistance and the mil-foot resistance. 

34. Specific Resistance. — If the length of a conductor 
one centimeter and its cross-section one square centimeter, 
then, by equation (32), we have R = K; that is, (K) is the 
resistance of a cubic centimeter of the material considered. 
If the length of the conductor is one inch and the area one 
square inch, then (K) is equal to the resistance of a cubic 
inch of the material. The resistance of a cubic centimeter 
or cubic inch of any material is called the specific resist- 
ance of the material, and it is usually expressed in microhmj 
at 0° centigrade. (A microhm is the one-millionth part o: 
one ohm. 

35. Circular Mil. — In the majority of cases electrical con- 
ductors have a circular cross-section and when we calculate 
the cross-sectional area from the diameter, the awkward fac- 
tor .7854, or (tt -^ 4), appears. .To avoid this factor a more con- 
venient practical unit of area has been adopted — the circular 
mil (cm.). The circular mil is the area of a circle whose 



CALCULATION OF RESISTANCE 



29 



diameter is one mil or .001 of an inch. The square mil, 
on the other hand, is the area of a square whose side is one 
mil. The advantage in the use of the circular mil is that 
when the diameter of the conductor is given in mils, its cir- 
cular-mil area may be determined by squaring the diameter. 
Conversely, the diameter may be determined, when the cir- 
cular-mil area is given, by taking the square root of the 
area. This gives the diameter in mils and from it the diame- 
ter in inches may be obtained by dividing, by one thousand. 

36. Mil-Foot Resistance. — The mil-foot resistance of a ma- 
terial is defined as being the resistance of a volume of the 
material one foot in length and having a uniform cross-sec- 
tion of one circular mil. Then if (Z) is equal to one foot 
and (A) is equal to one circular mil, in equation (32) (K) 
will be equal to (R). The mil-foot resistance is really a 
particular value for the specific resistance, as it is the re- 
sistance of a certain volume. 

Values of Specific and Mil-Foot Resistance. — The values 
of the specific resistances of some of the common materials 
are given in Table No. IV. 



TABLE NO. IV 

SPECIFIC RESISTANCE, PER CENT RELATIVE RESISTANCE AND 
CONDUCTANCE, OF DIFFERENT MATERIALS 





Measurements at 0** Centigrade 


Relative 
Resist- 
ance % 


Relative 


Material 


Microhms 

per cubic 

cm. 


Microhms 

per cubic 

inch 


Mil- 
foot 
res. 


Conduc- 
tivity % 


Copper (Matthies- 
sen's Standard) 
Copper, annealed,. 
Silver 


1.594 
L56 
L47 
5.75 
9.07 
20.4 

8.98 
94.3 

2.> 

2. 6 


.6276 
.614 
.579 

2.26 

3.57 

8.04 

3.53 
37.1 

.865 

1.01 


9.M 
9.35 
8.82 
34.5 
54.5 
123. 

53.9 
566. 

13.2 

15.4 


100.0 
97.5 
92.5 

362. 

570. 
1280. 

565. 
5930. 

138. 

161. 


100.0 
102.6 
108.2 


Zinc (pure) 

Iron (very pure).. 

Lead (pure) 

Platinum (an- 
nealed) 


27.6 
17.6 

7.82 

17.17 


Mercury: 


1.69 


Gold (practically 
pure) 


72.5 


Al u m i n u m (99% 
pure) 


62.1 











30 PRACTICAL APPLIED ELECTRICITY 

The above values are based upon Matthiessen's determina- 
tion of what he thought to be the specific resistance of pure 
copper. The copper he used, however, contained considerable 
impurities and as a result the copper obtainable at the pres- 
ent time has a specific resistance less than that determined 
by Matthiessen in 1860. 

37. Relative Conductivity. — The conductivity of a mate- 
rial is equal to the reciprocal of its specific resistance. The 
relative conductivity of any material would be the percent- 
age relation between the conductivity of the material and 
the conductivity of copper, which is taken as the standard: 

Conductivity of material 

% relative conductivity = X 100, or 

Conductivity of copper 

Specific resistance of copper 
= X 100 (33) 



Specific resistance of material 



The value of the per cent relative conductivity of a number' 
of different materials is given in Table No. IV. 

38. Relative Resistance. — The relative resistance of a ma- 
terial in per cent is the relation between its specific resist- 
ance and the specific resistance of the standard, multiplied 
by 100: 

% relative Specific resistance of material 

resistance = ' X 100 (34) 

Specific resistance of standard 

The value of the per cent relative resistance of a number 
of different materials is given in Table No. IV. 

39. Relation between Square and Circular-Mil Measure. — 
In the calculation of the resistance of electrical conductors 
it is often necessary to change from the square to the circu- 
lar mil or vice versa. The relation of the area represented 
by one circular mil as compared to the area represented 
by one square mil can be easily shown by reference to Fig. 5. 
The small square in the figure represents an area corre- 
sponding to 100 square mils (these areas are greatly exag- 
gerated in the figure). A circle drawn inside the square as 



CALCULATION OF RESISTANCE 



31 



shown will have an actual area less than the square. The 
area of the square in square mils is equal to the product 
of the two sides measured in mils, or it is the value of the 
side in mils squared. The circle has an area in square mils 
equal to the diameter squared times .7854, or the area of 
the circle is .7854 of the area of the square. Now the area 
of the circle in circular mils (by definition of the circular 
mil, Sec. 35) is equal to the diameter in mils squared (d)2. 

The number of square mils en- 
closed by the circle will then be 
equal to .7854 of the number of 
circular mils enclosed by the circle, 
or the actual area corresponding to 
a circular mil is less than the area 
corresponding to one square mil. 
This results in there always being 
a greater number of circular mils 
in any area than there are square 
mils. To change from circular to 
square mils, multiply the area in 
circular mils by .7854 and the result 
is the area in square mils. Or, to 
change from square to circular mils, divide the area in square 
mils by .7854 and the quotient thus obtained will be the area 
in circular mils. 

Example. — The diameter of a circular copper conductor 
is 102.0 mils. Determine the area of the above conductor in 
both circular and square mils. 

Solution. — The area of any circular conductor in circular 
mils is equal to the diameter of the conductor in mils, 
squared, or 




Fig. 5 



Circular-mil area == d2 = (102.0)2 == 10 404 

Ans. 10 404 circular mils. 

(2) The area of a circle in square measure is equal to 
.7854 times the diameter of the circular squared, or 

Square mil area = .7854 X d^ = .7854 X (102.0)^ = 8171.3 

Ans. 8171.3 square mils. 



40. Calculation of Resistance from Dimensions and Spe- 
cific Resistance. — The resistance of any conductor may be 



32 PEACTICAL APPLIED ELECTKICITY 

calculated by the use of equation (32) if the dimensions of 
the conductor and the value of (K) are known. The value 
of the constant (K) used in the equation will, of course, 
depend upon the units used in expressing the length and 
area of the conductor. When the length (0 of the conductor 
is expressed in feet and the area (A) in circular mils, the 
constant (K) corresponds to the mil-foot resistance of the 
material. The value of this constant for different materials 
can be obtained from Table No. 11. The mil-foot resistance of 
commercial copper at 25° centigrade is approximately 10.8. 

41. Wire Gauges. — For many purposes it is desirable to 
designate the size of a wire by gauge numbers rather than 
by a statement of their cross-section. A number of wire 
gauges have been originated by different manufacturers of 
wire, such as the B. & S. gauge, commonly called the Amer- 
ican gauge. Brown and Sharpe Manufacturing Company, 
which is the one generally used in this country. In the meas- 
urement of iron and steel wire the "Birmingham wire gauge" 
(B. W. G.), or Stub gauge is usually used. There are a num- 
ber of other gauges such as the Roebling, Edison, and 
New British standard, but these are not used very much. 
The diameters in mils of the different size wires is given 
in Table D, Chapter 20. 

The B. & S. gauge is by far the most common wire gauge 
in use in this country and for that reason it will be used 
almost entirely in wiring calculations. Tables E and F, 
Chapter 20, give the properties of copper wire. 

42. How to Remember the Wire Table. — The wire table 
has a few simple relations, such that if a few constants are 
carried in the memory, the whole table can be constructed 
mentally with approximate accuracy. The chief relations, 
without proof, may be enumerated below and verified from 
the table. 

The following approximate relations should be remembered: 
No. 10 B. & S. gauge wire is 100 mils in diameter, approxi- 
mately; has an area of 10 000 cm.; has a resistance of one 
ohm per thousand feet; and weighs 31.43 pounds per thou- 
sand feet, at 20°C. (68°F.). No. 5 wire weighs 100.2 pounds 
per 1000 feet. The following rules are approximately true 
for B. & S. gauge wire. 



CALCULATION OF EESISTANCE 33 

(a) A wire which is three sizes larger than another has 
half the resistance, twice the weight, and twice the area. 

(b) A wire which is ten sizes larger than another has one- 
tenth the resistance, ten times the weight, and ten times 
the area. 

(c) To find the resistance, divide the circular-mil area by 
10; the result is the number of feet per ohm. 

(d) To find the weight per thousand feet, divide the 
number of circular mils by 10 000 and multiply by the weight 
of No. 10 wire. Table C, €hapter 20, gives the equivalent 
cross-sections of different size wires. 

PROBLEMS 

(1) What is the circular-mil area of a wire 14 inch in 
diameter? 

Ans. 62 500 cm. 

(2) The circular-mil area of a wire is 4225. What is its 
diameter in inches? 

Ans. .065 inch. 

(3) A certain rectangular piece of copper is % by i/^ of 
an inch in cross-section. What is the area of this bar in 
square mils? 

Ans. 125 000 sq. mils. 

(4) What is the area of the rectangular piece of copper 
given in problem 3, in circular mils? 

Ans. Approximately, 159 150 cm. 

(5) What would be the diameter of a circular con- 
ductor in mils that would have the same actual area as the 
rectangular piece of copper given in problem 3? 

Ans. Approximately 399. mils. 



CHAPTEE III 



SERIES AND DIVIDED CIRCUITS— MEASUREMENT OF 
RESISTANCE 

43. Grouping of Conductors. — The resistance of a whole or 
a portion of a circuib will depend upon the manner in which 
the various parts constituting the circuit are connected. 
Two or more conductors may be combined in a number of 
ways, and the total resistance of the combination can be 
determined if the resistance of each part of the circuit and 
the manner in which the various parts are connected is 
known. The various ways in which conductors may be 
grouped are as follows: 

(a) Series grouping. 

(b) Parallel or multiple grouping. 

(c) Any combination of series and parallel. 

44. Series Grouping. — 
When the conductors form- 
ing a circuit are so ar- 
ranged that the current has 
a single path the conductors 
are said to be connected 
in series and such a circuit 
is called a series circuit. 
Pig. 6 shows three resist- 
ances (Ri), (R2),and (R3), 
connected in series to the 
terminals of the battery 
(B). The current has only 
one path from the positive to the negative terminal of the 
battery. These resistances may be of widely different values 
and composed of different materials, but the total resistance 
of the combination is equal to the sum of the resistances of 
the various parts. 

Suppose the three resistances in Fig. 6 have values of 3, 
4, and 5 ohms, then the total resistance of the circuit, neg- 

34 




B 



B 



Fig. 6 



SERIES AND DIVIDED CIRCUITS 



35 



lecting the resistance of the connections and the internal 
resistance of the battery, will be 3 + 4 + 5—12 ohms. 

The above relation of the total resistance to the individual 
resistances can be shown by a hydraulic analogy. Suppose 
that in Fig. 7 (A), (B), and (C) are three pipes of different 
size and length and that they are joined end to end, or in 




Fig. 7 



series, and they are to be used in conducting water from 
the tank (Tj) to the tank (T2). It is apparent that the total 
resistance offered by the three pipes connected in series is 
equal to the sum of their respective resistances. 

45. Facts Concerning Series Circuit. — There are four facts 
concerning every series circuit: 

(a) The current is uniform throughout the series circuit. 

(b) The p.d.'s over any portions of the series circuit are 
proportional to the resistances of these portions. 

(c) The total resistance is the sum of the individual 
resistances. 

(d) The effective e.m.f. of the circuit is the algebraic 
sum of all the e.m.f.'s acting in the circuit. 

46. Uniformity of Current. — Since there is but one path 
for the current, the conductors being all joined in series, 
there must be as much current at one end of the conductor 
as at the other. If this were not the case there would be 
an accumulation of electricity at certain points along the cir- 
cuit, but careful experiments show no such accumulation. 
The flow of electricity can be compared to the flow of water 
or other incompressible fluid in a pipe, as shown in Fig. 7. 
If there is a certain quantity of liquid entering the end 
of the pipe, connected to the tank (T^), in a certain time, 



36 PEACTTCAL APPLIED ELECTRICITY 

then that same quantity must pass by any cross-section of 
the pipe and the same quantity must flow out of the other 
end of the pipe in the same time. It is impossible for the 
liquid to accumulate at any point as it is incompressible. 

An ammeter, which is an instrument used to measure the 
current, may then be connected at any point in a series cir- 
cuit and there will be the same indication on its scale so long 
as the total resistance of the circuit and the electrical pres- 
sure acting on the circuit remain constant. It must alwaysi 
be remembered that it is not the current in an electrical 
circuit that is used up, but instead, it is the energy of the 
electricity that is always utilized. 

47. Relation of P.D/s to Resistance. — Ohm's Law, which 
expresses the relation between electrical pressure, current, 
and resistance, holds for any part of the circuit as well 
as for the whole circuit. Consider a uniform conductor 



B CD 

Fig. 8 



(A, B, C, D), carrying a current of (I) amperes in the direction 
indicated by the arrow in Fig. 8. There must be a difference 
in pressure between the points, say (A) and (B), since there 
is a current between them and the point (A) is at a higher 
potential or pressure than the point B when the current 
exists in the direction indicated. Let the resistance between 
the points (A) and (B) be represented by (Ri) and the 
difference in pressure or drop in potential between the same 
two points be represented by (Ei). The current is then equal 
to (El) divided by (Ri), or 

El 
I = — (35) 

Ri 
If any other section of the conductor be taken, such as 
that between the points (C) and (D), and the drop in poten- 
tial between the two points be represented by (E2) and 
the resistance of the section by (R2) we have 

E2 
I = — (36) 

R2 



r 



SEEIES AND DIVIDED CIECUiTS 37 



'The currents in the two sections are equal since they are 
in series and the right-hand portions of the above equations 
are equal, hence 

xLj lljo 

— =-■— (37) 

Ki R2 

or the drop in potential is proportional to the resistance. 

Example. — Two coils of 5 and 10 ohms are connected in 
series to a battery whose e.m.f. is 6 volts. What is the poten- 
tial drop over each coil? 

Solution. — From equation (37) we can determine the rela- 
tion between (Ej) and (E2), where they represent the drops 
in potential over the two coils, by substituting the values 
of (Ri) and (R2) in the equation, which gives 

5 10 
5 E2 = 10 El 
E2 ^=^ ■" E^ 

The p.d. (E2) over the 10-ohm coil is then equal to twice 
the p.d. (Ei) over the 5-ohm coil. The 5-ohm coil will then 
have i/{> of the total pressure over it and the 10-ohm coil will 
have %. Since the total pressure is 6 volts, the drop over the 
5-ohm coil will be 

1/3 of 6 = 2 

Ans, 2 volts, 
and the p.d. over the 10-ohm coil will be 

% of 6 = 4 

Ans. 4 volts. 

48. Resistance of Series Circuit. — In a previous section 
the statement was made that the total resistance of the series 
circuit is equal to the sum of the various resistances com- 
posing the circuit. This statement is practically self-evident, 
but it may be shown to be true by an application of Ohm's 
Law. 

If a number of resistances (Ri), (Ro), etc., are connected 
in series and there is a current of (I) amperes through them, 
we have, from equation (35), 



38 PEACTICAL APPLIED ELECTRICITY 

Ei = RiI, Eo^RsI, etc., (38) 

where (Ei), (E2), etc., represent the p.d.'s over the resist- 
ances (Ri), (R2), etc. 

Let (R) represent the total resistance and (E) the total 
pressure. Then 

E = RI (39) 

But we also know (E) is equal to the sum of all the p.d.'s, 
that is, 

E = Ei + E2 -f etc. (40) 

Hence 

RI = R,I -h R2I + etc. (41) 

or 

R=-Ri + R2 + etc. (42) 

The above equation states that the resistance of several 
conductors joined in series is the sum of their individual 
resistances. 

Example. — Three resistance coils of 5, 6, and 7 ohms, re- 
spectively, are connected in series and the combination con- 
nected to a battery whose e.m.f. is 9 volts. What is the 
value of the current in the coils? 

Solution. — The total resistance (R) is equal to the sum 
of the several resistances, or 

R = 5 + 6 + 7 = 18 ohms 
The current then is equal to (E) divided by (R), or 

9 1 

18 2 

Ans. 1/^ ampere. 

If there are (n) equal resistances of (r) ohm each con- 
nected in series, the total resistance (R) is 

R = nr (43) 

An example of this would be a number of lamps connected 
in series and the combination then connected to the mains. 
If there are 5 incandescent lamps connected in series each 
having a resistance of 220 ohms, the combination will have a 
resistance of 5 X 220, or 1100 ohms. 

49. Effective E.M.F. in a Circuit. — The effective e.m.f. in 
a circuit is the e.m.f. that is really effective in causing the 
current. Several sources of e.m.f. may be joined in series. 



SERIES AND DIVIDED CIECUITS 



39 



but the effective e.m.f. would not necessarily be equal to the 
sum of their values because some of the e.m.f.'s might act 
in the opposite direction to others. If all the e.m.f.'s in 
the circuit tend to send a current in the same direction then 
the value of the effective e.m.f. is the sum of all the e.m.f.'s 
acting. When they are not all acting in the same direction, 
the effective e.m.f. is equal to the difference between the 
sum of the e.m.f.'s acting in one direction and the sum of the 
e.m.f.'s acting in the opposite direction, and its direction will 
be that of the larger sum. An example of the above would 
be a battery composed of a number of cells all connected in 
series but the e.m.f. of some of the cells acting in the oppo- 
site direction to the remainder. The effective e.m.f. is a 
maximum when all the e.m.f.'s in the circuit are acting in 
the same direction. 

50. Parallel or Multiple Grouping. — When the conductors 
forming a circuit are so connected that there are as many 
paths for the current as there are conductors, the conductors 
are said to be connected in parallel or multiple, and such a 

circuit is called a divided 
circuit. Fig. 9 shows three 
coils (Ri), (R2), and (R3) 
connected in parallel and 
the combination connected 
to the battery (B). In a 
circuit such as that shown 
in Fig. 9, it is apparent 
that the current cannot be 
the same in all parts of the 
circuit, since it divides at 
the point (A) between the 
branches, part existing in each branch. The part of the total 
current that is in each branch will depend upon the relation 
between the resistance of that particular branch to the re- 
sistance of the other branches. 

The total resistance is not equal to the sum of the several 
resistances, as in the series circuit, but it will be.Jess than 
the resistance of the branch having the smallest resistance. 
A simple hydraulic analogy, as shown in Fig. 10, will serve 
to verify the above statements. Three pipes, (P^), (P2), 
and (P3) are used in conducting water from a tank (Tj) 




Fig. 9 



40 



PKACTICAL APPLIED ELECTKICITY 




P, 



to another tank (T2), as shown in the figure. If the pressure 
in the tank (Ti) is maintained constant, the pressure acting 
on the three pipes will remain constant and it will be the 
same for each pipe, neglecting the difference in their level. 

The same pressure is act- 
ing on each of the resist- 
ances in Fig. 9, neglecting 
the resistance of the con- 
necting leads. Let this 
pressure be represented by 
(E). The current in each 
of the resistances will be 
equal to the presure over 
the branch, which in this 
case is (E), divided by the resistance of the branch. The 
current supplied by the battery is equal to the sum of the 
currents in the various branches, just as the total quantity of 
water flowing from the tank (Ti), Fig. 10, in a given time, 
is equal to the sum of the quantities flowing in the three 
pipes in the same time. Representing the currents in the 
several branches by (Ii), (I2), and (I3), and the total current 
by (I) we have the relation 






Fig. 10 



I = Ii + I2 + I3 

E E 



1.2 



Ri 



Ro 



E 



(44) 
(45) 



The total current (I) is equal to the electrical pressure 
(E) acting on the combination divided by the total resist- 
ance (R) which we want to determine, or 



E 



1 = - 



R 



(46) 



Substituting the values of the various currents in equation 
(44), we have 

E E E E 

— -— + — + — (47) 



SEKIES AND DIVIDED CIRCUITS 41 

Dividing both sides of the equation by (E), we have 

1111 

— = — + — + -- (48) 

R Ri Ro R3 

The above equation states that the total conductance of a 
number of resistances in parallel is equal to the sum of the 
respective conductances regardless of the number connected. 
This equation can be reduced to the form 

R1R2R3 
R = (49) 

R1R2 + ^2^3 + R1R3 

When there are only two resistances connected in parallel, 
the combined resistance can be calculated by the use of the 
equation 

R1R2 

R-= (50) 

Ri + R2 

If the resistances (Ri) and (R2) are equal, the combined 

resistance is equal to one-half of the resistance of either 

of them. Or, in general, if (n) equal resistances of (r) ohms 

each are connected in parallel, the combined resistance (R) is 

r 
R = — (51) 

n 
Example. — Eight incandescent lamps, each having a resist- 
ance of 220 ohms, are connected in parallel across a 110-volt 
circuit. What is the total current taken by the lamps? 

Solution. — The total resistance of the eight lamps would be 
equal to the resistance of one of them divided by the number 
of lamps connected in parallel, or 
r 220 

R = — = = 271/^ ohms 

n 8 
The current taken by the lamps is equal to the applied 
voltage divided by the resistance, or 

E 110 

R 271/2 

Ans. 4 amperes. 



42 



PRACTICAL APPLIED ELECTRICI'M^ 



Example. — Two resistances of 3 and 5 ohms, respectively, 
are connected in parallel. What is their combined resist- 
ance ? 

Solution. — The combined resistance can be determined by 
substituting directly in equation (50), which gives 

3X5 15 

R = = — =178 

3 + 5 8 

Ans. 1% ohms. 

51. Series and Parallel Combinaiions. — A number of re- 
sistances may be connected in such a way that some of them ji 
are in parallel with others, or some of them may be in series | 
with others, or a combination of series and parallel connec- i 
tlons may be formed. Six resistance coils are shown con- ] 

nected together in Fig. 11. ! 
This is a parallel combina- 
tion of three different re- 
sistances, the first of 
which consists of the re- 
sistances (R4) and (R5) 
in series; the second con- 
sists of (Ri), (R2), and 
(R3) in series; and the 
third consists of the single 
resistance (Re). To find 
the total resistance of the 
circuit determine the resistance of each path separately and 
combine their resistances by the use of equation (49). 

Several such groups of resistances may be connected in 
series and the total resistance would be the sum of the 
resistances of the several groups. 

Example. — The six coils shown connected in Fig. 11 have 
the resistances marked on them in the figure. What is the 
total resistance? 

Solution. — The resistance of the upper branch, call it (B^). 
is equal to the sum of the resistances (R4) and (R5), these 
being connected in series, or 

Bi = 3 + 4 = 7 ohms 
Similarly the resistance of the middle branch (B2) is 
B2 = 21/2 4-2 + 31/0 = 8 ohms 




-wwvww 

6 

Fig. 11 



SEEIES AND DIVIDED CIECUITS 43 

and the resistance of the lower branch (B3) is 6 ohms. Sub- 
stituting these values for the resistances of the various 
branches in equation (49), we obtain 

7X8X6 336 

R = = = 2.301 

7X8 + 7X6 + 8X6 146 

Ans. 2.301 ohms. 

PROBLEMS ON SERIES AND DIVIDED CIRCUITS 

(1) Five similar incandescent lamps are connected in 
series across 550-volt mains, and there is a current of .5 
ampere through them. What is the combined resistance of 
the five lamps and the resistance of each? 

Ans. Combined resistance, 1100 ohms. 
Resistance of each lamp, 220 ohms. 

(2) An adjustable resistance is connected in series with 
the field winding of a dynamo, which has a resistance of 40 
ohms, and a current of 2 amperes exists in the circuit when 
the impressed voltage is 110 volts. What is the total resist- 
ance of the circuit, and how many ohms resistance is there 
in the circuit due to the adjustable resistance? 

Ans. Total resistance, 55 ohms. 

Adjustable resistance, 15 ohms. 

(3) There is a current of 50 amperes in a circuit when the 
impressed voltage is 200 volts. What resistance should be 
added in series with the circuit in order that the current be 
reduced to 40 amperes? 

Ans. 1 ohm. 

(4) What resistance should be connected in parallel with 
the circuit in problem 3 when the current is 50 amperes in 
order that the total current may be 80 amperes? 

Ans. 6% ohms. 

(5) Three resistances of 5, 6, and 7 ohms, respectively, are 
connected in parallel. What is their combined resistance? 

Ans. 1.96 ohms. 

(6) If 12 similar incandescent lamps connected in parellel 
have a combined resistance of 18% ohms, what is the resist- 
ance of each lamp? 

Ans. 220 ohms. 



44 PRACTICAL APPLIED ELECTEICITY 



(7) Two resistances of 4 and 12 ohms, respectively, are 
connected in parallel and the combination connected in series 
with a 7-ohm coil. What is the current through each resist- 
ance when a pressure of 50 volts is impressed upon the 
circuit? 

Ans. Current in 7-ohm coil, 5 amperes. 
Current in 12-ohm coil, l^i amperes. 
Current in 4-ohm coil, 3% amperes, 

(8) What is the drop in potential over each resistance in 
the above problem? What resistance should be introduced in 
the circuit, and how, to make the value of the current 2.5 
amperes? 

Ans. Drop over 7-ohm coil, 35 volts. 
Drop over 4-ohm coil, 15 volts. 
Drop over 12-ohm coil, 15 volts. 
Connect 10 ohms in series. 

52. Measurement of Resistance. — Practically all the meth- 
ods employed in measuring resistance depend upon some ap- 
plication of Ohm's law. The method to be used in any case 
will depend upon the kind of resistance to be measured, 
that is, whether it is capable of carrying a large or small cur- 
rent; the accessibility of the resistance; the value of the 
resistance to be measured; and the accuracy desired. The 
different methods described in the following sections are, per- 
haps, the most common ones employed in practice. 

53. Drop in Potential iVIethod. — Since Ohm's Law holds 
true for any part of an electrical circuit, it follows that 
the value of a certain resistance can be determined by measur- 
ing the drop in potential across the resistance when the cur- 
rent through the resistance is known. A voltmeter for meas- 
uring the drop in potential, an ammeter for measuring 
the current, and some source of energy such as a battery or 
generator are required in measuring resistance by this method. 
The resistance (R) to be measured is connected in series with 
the ammeter (A), and the combination then connected to 
the source of energy such as a battery (B), indicated in Fig. 
12. The drop in potential across the resistance can be deter- 
mined by means of the voltmeter (V), which should be 
connected to the terminals of the resistance, as shown in the 
figure. The value of the resistance can be determined by 



I 



SEEIES AND DIVIDED CIKCUITS 



45 



substituting the ammeter and voltmeter readings in equation 
(5), which states that the resistance is equal to the differ- 
ence in electrical pressure divided by the current. This is a 
very simple and convenient method and will give quite accur> 




Fig. 12 

ate results when proper care is exercised in reading the in- 
struments. The method is best suited for the measurement of 
low resistances capable of carrying rather large currents, [or 
the following reason : The current in the resistance to be meas- 
ured is not equal to that indicated on the ammeter because 
there is a certain current through the voltmeter. The current 
through the voltmeter will, however, be small in comparison 
to that through the unknown resistance, if the resistance of 
the voltmeter is large in comparison to the unknown, the cur- 
rents in the two branches of a divided circuit being to each 
other inversely as the resistances of the respective branches. 
This error can be avoided by subtracting from the value of 
the current indicated on the ammeter the value of the cur- 
rent through the voltmeter, which gives the true value of the 
current through the resistance to be measured. The current 
through the voltmeter is equal to its indication in volts 
divided by its own resistance, or 

E- 
Iv==— (52) 

Rv 

The resistance of the voltmeter is usually given on the 
lid of the containing case. If not, it can be determined by 



46 PEACTICAL APPLIED ELECTRICITY 

means of a resistance bridge. The current Ir through the 
resistance then is 

Ir = Ia — Iv or (53) 

Ir = Ia (54) 

Rv 

and the value of the unknown resistance will be 

E E E 
R = ~ = = R^ (55) 

Ir E laRv — E 

la 

Rv 

Care should be exercised in making a resistance measure- 
ment by this method not to pass too large a current through 
the object to be measured as you are likely to change its 
resistance due to a change in temperature resulting from 
the excessive current. The greater the current, however, 
without undue heating of the conductor, the greater the volt- 
meter reading and, as a usual thing, the greater the accuracy. 
When very low resistances are to be measured, a low-reading 
voltmeter or, better still, a millivoltmeter should be used. 

Example.— The current through a rail joint is 300 amperes 
and the drop in potential across the joint and bonds is 18 
millivolts or .018 volt. What is the resistance in ohms and 
microhms? 

Solution.— By substituting in equation (5) we have 

.018 

R = =.000 06 

300 

Ans. .000 06 ohm. 

0.000 06 X 1 000 000 = 60 

Ans. 60 microhms. 

Example.— The current through a spool of wire is .5 am- 
pere and the drop in potential across the spool as indi- 
cated on a (0-15) voltmeter, having a resistance of 1500 ohms, 
is 12 volts. What is the resistance of the wire? 

Solution.— Substituting in equation (55) we have 



SEKIES AND DIVIDED CIRCUITS 



47 



12 



R: 



X 1500 = 24.39 



.5 X 1500 — 12 



Ans. 24.39 ohms. 
54 Measurement of Resistance by Comparison. — This 
method requires no ammeter and the value of the unknown 
resistance (X) is determined in terms of a known resistance 




JWWVWr 



Fig. 13 



(R) connected in series with it, as shown in Fig. 13. The 
drop in potential over the known and unknown resistances is 
measured when they are both carrying the same current. 
The proper connections of the voltmeter for making these 
measurements is shown in the figure by the full and dotted 
lines. Since the drop in potential across any part of a cir- 
cuit bears the same relation to the drop across any other part 
Of the same circuit as exists between the resistances of the 
two parts, we have the simple relation 

Drop in potential over X Resistance of X 



Drop in potential over R 



(56) 



Resistance of R 



o-r 



the unknown Resistance of R X p. d. over X 
resistance X = = 



(57) 



p. d. over R 
Example. — A known resistance (R) of 10 ohms was con- 
nected in series with an unknown resistance (X), as shown in 
Fig. 13. The drop in potential over (R) was 5 volts and the 
drop over X was 10 volts. What was the resistance of (X) ? 



48 



PEACTICAL APPLIED ELECTEICITY 



Solution.— Since the drop over the resistance (X) was twice 
that over the standard, the resistance of (X) must be twice 
that of the standard, or 

X = 2 X 10 = 20 

Ans. 20 ohms. 
Substituting in equation (57) we have 

10 X 10 
X=: = 20 



Ans. 20 ohms. 
55. Series Voltmeter Method.— The connections for the 
measurement of resistance by this method are made as 
shown in Pig. 14. The terminals (Ti) and (T2) represent 

a source of e.m.f.; (X) is 



T. T 




the resistance to be meas- 
ured and (V) is a direct 
reading voltmeter. When 
the voltmeter is con- 
nected as shown by the 
dotted line, the resistance 
(X) is not in series with 
it between the terminals 
(Ti) and (T2) and the 
voltmeter indicates the 
., ^ .„,^ , . , „„ *^^^^ pressure between 

the two terminals. When, however, the resistance (X) is con- 
nected in series with the voltmeter by opening the switch 
(S), the indication of the voltmeter is no longer the total 
pressure between the terminals (T^) and (T^) but it sim- 
ply indicates the difference in pressure between its own 
terminals. The drop in potential over the voltmeter (Ev) 
or Its own indication subtracted from the total pressure be- 
tween (Ti) and (T,), which we will call (E), will give the 
value of the drop (EJ over the resistance (X). 



E. 



E — Ev 



(58) 



It is assumed, of course, that the difference in pressure 
between (T,) and (To) remains constant. If this is not the 
case a second voltmeter should be used, it being connected 
across the source of supply, and the value of its indication 



SERIES AND DIVIDED CIRCUITS 49 

should be noted at the same time the voltmeter in series with 
the resistance is read. The current through the voltmeter 
and the resistance (X) is the same, since they are in series. 
The current through the voltmeter at any time is equal to the 
voltmeter reading divided by its own resistance, or 

Ev 
Iv = — (59) 

Rv 

The value of the resistance (X) can now be calculated, 
since the current through it and the drop in potential across 
it are known. Substituting in equation (5) the values of the 
p.d. and the current given in equations (58) and (59), gives 

E — Ev E — Ev 

.R = • = X Rv (60) 

Ev Ev 

Rv 

The above equation gives the value of the resistance in 
terms of the total pressure (E), the voltmeter reading (Ev) 
when it is in series with the resistance, and the resistance 
(Rv) of the voltmeter. 

This method is, in general, serviceable for the measure- 
ment of high resistances, such as the insulation of electric 
light and power wires that are installed, insulation of trol- 
ley lines, dynamos, transformers, etc. 

The scheme of connections for the measurement of the 
resistance of the insulation of a lighting circuit is shown in 
Fig. 15. A small generator (G) capable of supplying an 
e.m.f. of about 500 volts is usually used as a source of pres- 
sure for testing. The generator may be engine- or motor- 
driven. One terminal of the generator is connected directly 
to the conduit or ground and the other terminal is connected 
to the wire whose insulation resistance is to be determined, 
with the voltmeter in circuit. The total pressure generated 
by the machine (G) will be distributed over the resistance 
between the wire and the conduit, or ground, and the volt- 
meter. The voltmeter will read the drop (Ev) across itself. 
A second voltmeter (Vg) is shown connected across the ter- 
minals of the generator, and this voltmeter will read the 
total pressure (E). The value of the insulation resistance 



50 



PRACTICAL APPLIED ELECTRICITY 



(X) can now be calculated by substituting the values of the 
readings of the two voltmeters in equation (60) together with 
the resistance of the voltmeter (Vx) and solving the equa-j 
tion. Insulation resistance is usually given as so many| 
megohms. 




0-500 




0-500 
G ) Generator 



Conduit 




I Ground 

Fig. 15 



Example. — Connections were made, as shown in Fig. 15, 
for testing the insulation resistance of a certain electric 
light system. The resistance of the voltmeter (Vx) connected 
in series with the resistance to be measured was 50 000 ohms. 
The voltmeter (Vg) read 500 volts and (Vx) 10 volts. What 
was the insulation resistance in megohms? 

Solution. — Substituting the voltmeter readings and the re-| 
sistance Rv in equation (60) gives 



500 — 10 



R = 



X 50 000 = 2 250 000 



10 



2 250 000 -- 1 000 000 = 2.25 
Ans. 



2.25 megohms. 



56. Direct-Deflection Method. — A galvanometer (G) is con- 
nected in series with the resistance to be measured and the 
two then connected to a source of e.m.f., as shown in Fig. 
16. The indication produced on the galvanometer when 
the circuit is closed should be noted. The unknown resist- 
ance is then replaced by a known resistance and the cir- 



SEKIES AND DIVIDED CIKCUITS 



51 



cuit again closed and the deflection again noted. The cur- 
rent in the circuit for the two cases can be determined 

from the deflections of the 
galvanometer. Let (Rg) 
represent the resistance of 
the galvanometer, (EX the 
battery pressure, (R) the 
known resistance, (X) the 
unknown resistance, (Ir) 
^^^' ^^ the current through the 

known resistance, and (Ix) 
the current through the unknown resistance, then 




E 



and 



Rg + X 



(61) 



E 



Ir = 



Rg + R 



(62) 



Since the same pressure is acting on the circuit in both 
cases, and knowing the current in a circuit will vary in- 
versely as the resistance, the relation may be written 

Ix Rg + R 

— = (63) 

Ir Rg + X 

Calculating the value of (X) from the above equation we 
have 
Tj Ir (Rg + R) 

X=== Rg (64) 

Ix 

The resistance of the galvanomete];' is usually very small 
in comparison to the resistance being measured so that it may 
be neglected in the above equation, which gives 

Ir 

X = — XR (65) 

Ix 

as the resistance of the unknown in terms of the current 
in the two cases and the value of the known resistance (R). 



52 



PRACTICAL APPLIED ELECTRICITY 



This method is used in determining the insulation of coils of 
wire, etc. The wire whose insulation is to be measured is 
immersed in a salty solution (it being a better conductor than 
ordinary water), with at least three feet of the wire out of the 
solution at each end. One terminal of the testing circuit is 
connected to the wire itself and the other terminal to a 
metallic plate placed in the solution. The resistance between 
the wire and the solution is then measured, giving the insula- 
tion resistance for the length of wire immersed in the solution. 

The insulation resistance of a wire varies inversely as its 
length, because with an increase in length there is more 
surface exposed, resulting in a greater leakage and less 
resistance. 

57. Principle of the Slide-Wire Wheatstone Bridge. — Two 
resistances, which may be equal or unequal, are shown con- 
nected in parallel between the points (A) and (B), Fig. 17, 
and the combination is connected to a source of e.m.f., such 



rAA/WVWWWWVWWVWNri 

A. ^^ Q 

^A/WWWWWWWWWWW 
B c 9- K == 



I'l^ 



Fig. 17 



as the battery (B). The drop in potential across the two 
branches of the divided circuit is the same regardless of the 
relation of the two resistances. If the resistance in each 
branch is the same, then the drop over a certain resistance in 
one branch is equal to the drop over the same resistance in 
the other branch. When the resistances of the two branches 
are not equal, the above relation does not hold true, but there 
are points on the two branches that have the same potential 
with respect to (A) or (B). Select some point, such as (C) 
on the lower branch, that will always have a potential less 
than the point (A) and higher than the point (B) when the 
current is in the direction indicated by the arrows. There is 



SEEIES AND DIVIDED CIECUITS 53 

a point on llie upper branch whose potential is equal to that 
of (C) and this point can be located by means of a galvanom- 
eter as follows: Connect one terminal of the galvanometer to 
the point (C) and slide the other terminal along the upper 
branch until a point is found which results in no deflection 
of the , galvanometer when the circuit is closed. This point, 
which is marked (D) in the figure, is at the same potential as 
the point (C), since there is no current between them, there 
being no deflection produced on the galvanometer. 

When the point (D) has been located, the drop in potential 
across (AC) is equal to the drop across (AD), and the 
drop across (C E) is equal to the drop across (DB). The 
resistance (AC) bears the same relation to (AD), after a 
balance is obtained, as the resistance (CB) bears to (DB). 
This statement may be put into the form of a simple equa- 
tion, thus 

Resistance (AD) Resistance (DB) 

= (66) 

Resistance (AC) Resistance (CB) 

For example, suppose the resistance (A C) and the total 
resistance of the upper branch are known and the resistance 
(CB) is unknown. A balance is obtained, as previously de- 
scribed, and, from the position of the point (D) on the upper 
branch, the values of the resistances (AD) and (DB) may 
be determined, the combined resistance of the two being 
known. The above equation can then be changed to the form 

Res. (DB) 
Resistance (C B) = X Res. (A C) (67) 

Res. (AD) 

By substituting the value of the three resistances (AC), 
(DB), and (AD) in the above equation, the value of the 
resistance (C B) may be determined. This type of bridge is 
called a slide-wire pattern because the upper branch is usually 
a piece of resistance wire stretched between the points (A) 
and (B). The wire is stretched over a board divided into 
equal parts and the relation between the two resistances 
(AD) and (DB) can be determined in terms of their respect- 
ive lengths, the resistance of the wire varying directly as the 
length. The two resistances (A D) and (D B) are called the 



54 



PRACTICAL APPLIED ELECTRICITY 



ratio-arms, since their relation to each other gives the ratiq 
between the known and the unknown resistances. 

58. Commercial Wheatstone Bridge. — The form of Wheatj 
stone bridge described in the previous section is not used 
to any great extent in practice, as its operation is usually tocT 
tedious, and for this reason it is confined almost entirely to 
laboratory work. The principle of the commercial bridge is 
the same as that of the slide-wire bridge, differing only in 
construction and operation. A diagram of a simple form of 




Fig. 18 



bridge is shown in Fig. 18. The letters in the figure corre- 
spond to those in Fig. 17. The resistances (DB) and (DA) 
are the ratio arms, consisting of three coils each, having the 
resistances marked in the figure. The resistance (AC), 
called the rheostat of the bridge, consists of a number of coils 
ranging in value from a very low resistance to several hun- 
dred ohms, depending upon the range of the bridge. The 
various resistance coils that form the different arms of the 
bridge can be cut in or out of circuit by means of metallic 
plugs that connect massive brass or copper strips on top 



SERIES AND DIVIDED CIRCUITS 55 

of the bridge. When a certain plug is removed the resistance 
coil that was shorted by the plug is connected in the circuit. 
The unknown resistance (X) is connected between the points 
(B) and (C). When a balance is obtained on the galvanom- 
eter, the following relation exists between the various arms of 
the bridge 

a R 

— -— (68) 

or b X 

b 
X = — R (69) 

a 

In making a measurement with this form of bridge the 
relation between the ratio arms (a) and (b) remains con- 
stant after they are adjusted to a certain value, and a balance 
is obtained by changing the value of the resistance in the 
rheostat (R). If (a) and (b) are made equal, then the resist- 
ance in (R), when a balance is obtained, is equal to the 
resistance of the unkrown (X). When it is desired to measure 
a resistance larger than the value of (R), make the ratio arm 




Fig. 19 



(b) greater than (a); and to measure a resistance smaller 
than (R), make (b) less than (a). In the first case the resist- 
ance (R) is multiplied by a certain number, which is the 
quotient of (bn-a), and will always be greater than imity 
to obtain the value of (X); and in the second case (R) will 
be multiplied by a number less than unity. The galvonometer 



56 



PKACTICAL APPLIED ELECTRICITY 



and the battery connections may be interchanged without 
interfering with the operation of the bridge. 

There are a large number of different forms of bridges on 
the market at the present time. Some of them have the gal- 
vanometer, battery, and contact keys all mounted in the same 
box with the resistances, and the only connection that must be 
made is to the resistance to be measured. Fig. 19 shows a 
good form of portable Wheatstone bridge. 

59. The Ohm meter. — An Ohmmeter is an instrument for 
measuring automatically the resistance of a circuit connected 
to its terminals, by noting the position of a pointer on a dial 
that is marked to read directly in ohms. The principle of 
the Evershed instrument is as follows: The deflecting sys- 
tem consists of a set of coils (BBi), as shown in Fig. 20, 
rigidly fastened together, which move about a center (O) in 
the magnetic field of strong permanent magnets (MM). The 
nature of this construction brings the instrument into the 
class of moving-coil permanent-magnet instruments, the ad- 
vantages of which for reliability, accuracy under all condi- 
tions of use, and promptness in taking deflection, are quite 















M 




A 


^^^C 






B 




B 


J 






E 


R 


f\ 






y^ 














M 

































Fig. 20 



numerous. Springs are not used for the control of the moving 
system, so that when not in use the needle may stand at any 
point along the scale. 

If the generator is set in motion and no resistance is con- 
nected between the external terminals, current exists only in 
the coils (BBi), which move at once to such location that (B) 
is clear of the horn on the pole piece and (Bj) is central 
over the air gap in the (O) shaped centrally placed hollow 



SEEIES AND DIVIDED CIRCUITS 57 

iron cylinder. The needle then stands over Uif, on the scale, 
showing an infinite resistance between the terminals. If a 
measurable resistance is connected between the terminals, 
a current exists in the stationary coil and, due to the thrust 
experienced by it in the magnetic field, the needle moves along 
the scale, and a direct reading of the amount of resistance 
so connected is made. 

The hand dynamo (D) operates in the field of the same per- 
manent magnets. The armature is wound so that a rated 
e.m.f. of 125, 250, 500, or even 1000 volts are generated for 
some 100 r.p.m. of the crank. The design of this machine 
is the result of a large amount of research by Mr. Evershed, 
in his desire to make a rugged, reliable, light-weight gen- 
erator which v/ould require slight propelling force to drive it. 

Instruments of this kind are usually used in measuring very 
high resistances, such as insulation resistances, etc. 



CHAPTEE IV 

PRIMARY BATTERIES 

60. The Voltaic Cell. — If two unlike metals are immersed 
in a solution, which is capable of acting upon one of them 
more than upon the other, there will be a current of elec- 
tricity between them when they are connected by a wire. 
Such a combination constitutes a voltaic cell. This cell was 
first discovered by an Italian physicist, Volta, in 1800, and 
was named after him. It is, however, often called a gal- 
vanic cell, after Galvani, who was Volta's contemporary. 

61. Simple Voltaic Cell. — Two 
pieces of metal, such as copper and 
zinc, immersed in a solution that con- 
tains a little sulphuric acid, or other 
oxidizing acid, forms a simple voltaic 
cell. Such a cell is shown in Pig. 21. 
This cell is capable of furnishing a 
continuous flow of electricity through 
a wire whose ends are brought into 
contact with the strips of copper 
and zinc. When the electricity flows, 
the zinc is wasted away, its con- 
sumption furnishing the energy re- 
quired to drive the current through 
the cell and the connecting wire. 
The cell might be thought of then 
as a chemical furnace in which the fuel is zinc. The copper 
strip from which the flow starts in passing through the 
external circuit is called the positive pole of the battery, and 
the zinc scrip is called the negative pole. These poles are 
usually designated by the plus ( + ) and negative ( — ) signs. 

58 




Fig. 21 



PRIMARY BATTERIES 



59 



It is the difference in electrical pressure between the positive 
and the negative poles of the battery that causes a current in 
the circuit when the poles are connected. 

62. Voltaic Battery. — If a number of voltaic cells are joined 
in series — the zinc plate of one joined to the copper plate of 
the next, and so on — a greater difference in electrical pressure 
will be produced between the copper pole at one end and 
the zinc pole at the o-ther end. When the poles forming 
the terminals of such a series are joined, there will be a more 
powerful current than one cell would cause. [It is assumed 
that the resistance of the circuit connecting the two poles or 
terminals is practically the same as the resistance of the cir- 
cuit connecting the terminals of a single cell.] Such a group- 
ing of voltaic cells is called a voltaic battery. Four single cells 
(Ci), (C2), (C3), and (C4), are shown connected in series in 
Fig. 22. The cells may be combined in other ways and these 



z 


c 


__z 


c 




L 


C 




z 


c 


1 




:) c 


It 




J(tl 




J^ 


a 






i 






1 






M 


jk 


5( 


J 




! 


^fl 




'V 


^ Hiy 




1" 


if 








i^ 


-J 




is 


nI: 






iJt 



Fig. 22 



C, C2 C3 C4 

Fig. 23 



methods will be taken up later. It is customary to represent 
a single cell by two parallel lines, as shown in Fig. 23, 
instead of drawing a picture of the cell each time you want to 
show it in a diagram. The long line corresponds to the plus 
(+), or positive, terminal, and the short line corresponds to 
the minus (— ), or negative, terminal. 

63. Chemical Action in a Battery. — A continuous potential 
difference is maintained between the zinc and copper in a 
simple voltaic cell chiefly by the action of the exciting liquid, 
say sulphuric acid, upon the zinc. Sulphuric acid is a complex 
substance in which every molecule is made up of a group of 
atoms, 2 of hydrogen, 1 of sulphur, and 4 of oxygen; or in 
symbols H2SO4. The SO4 part of the acid has a very strong 
affinity for the zinc, and attacks it, when the plates are con- 
nected, producing a current, and forms zinc sulphate ZnS04, 
which is dissolved in the water. There will be two parts of 



60 



PEACTICAL APPLIED ELECTEICITY 



hydrogen gas liberated for every portion of the SO4 part o 
the sulphuric acid that unites with the zinc. The zinc thus 
replaces the hydrogen in the acid, when the cell is being 
used, setting the hydrogen free. This chemical reaction ii 
expressed in the equation 

Zn + H2SO4 = ZnS04 + H2 

Zinc and sulphuric acid produce zinc sulphate and hydrogen. 



I 



I' 



This chemical action continues as long as the battery is sup- 
plying a current, the zinc gradually wasting away and the 
power of the acid to attack the zinc gradually becoming 
exhausted. Electrical energy is thus supplied to the external 
circuit by the combination of zinc and acid inside the cell. 

64. Local Action. — When the circuit of a battery is not 
closed, the current cannot exist, and there should be no chem- 
ical action as long as the battery is producing no current. 
Ordinary commercial zinc, however, contains many impurities, 
such as tin, arsenic, iron, lead, carbon, etc., and these numer- 
ous foreign particles form local voltaic cells on the surface of 
the zinc inside the cell, with the result that the zinc is being 
continuously eaten away whether the cell is supplying current 
to an external circuit or at rest. These small cells weaken the 
current the main cell is capable of supplying under proper 
conditions. Often local action is caused by a difference in 
density of the liquid at different parts of the cell. This 
causes the zinc at the top of the cell to waste away and it 
may be entirely eaten off. 

65. Amalgamation. — To do away with this local action, and 
thus abolish the wasting of the zinc while the battery is at 
rest, it is usual to amalgamate the surface of the zinc plates 
with mercury. The surface to be amalgamated should be thor- 
oughly cleaned by dipping it into acid and then a few drops of 
mercury should be rubbed into the surface. The mercury 
unites with the zinc at the surface forming pasty amalgam. 
The foreign particles do not dissolve in the mercury, but float 
to the surface, and they are carried away by the hydrogen bub- 
bles. As the zinc in the pasty amalgam dissolves into the 
acid, the film of mercury unites with fresh portions of zinc, 
and a clean, bright surface is always presented to the liquid. 

66. Polarization. — When there is a current through a cell, 
the hydrogen that is liberated from the acid appears upon 



PRIMAEY BATTERIES 61 

the surface of the copper, and the copper plate becomes prac- 
tically a hydrogen plate. If a cell were made having a hydro- 
gen and zinc plate, there would be a current from the hydro- 
gen to the zinc inside the cell and from the zinc to the hy- 
drogen outside the cell. The hydrogen collecting on the copper 
plate tends to send a current through the cell opposite to that 
produced by the copper and zinc current. This results in the 
current supplied by the battery decreasing as the hydrogen 
on the copper plate increases, or the plate becomes more 
nearly covered with the hydrogen gas. A cell that has become 
weakened in this way is said to be polarized, and the phe- 
nomenon is called polarization. Hence, polarization is an evil, 
and if it could be overcome by preventing the hydrogen bub- 
bles collecting on the copper plate, the cell would be capable 
of supplying a current of almost constant strength as long as 
zinc remained to be acted upon and the acid was not ex- 
hausted. Various attempts to prevent polarization have given 
rise to many different types of cells on the market at the 
present time. 

67. Prevention of Polarization. — Various remedies have 
been practiced to reduce or prevent the polarization of cells. 
These may be classed as mechanical, chemical, and electro- 
chemical. 

(a) Mechanical Means. — If the hydrogen bubbles be sim- 
ply brushed away from the surface of the positive pole, the 
resistance they cause will be diminished. If air be blown 
into the acid solution through a tube, or if the liquid be 
agitated or kept in constant circulation by syphons, the re- 
sistance is also diminished. If the surface be rough or cov- 
ered with points, the bubbles collect more freely at the 
points and are quickly carried up to the surface and got 
rid of. This remedy is used in the Smee cell, which con- 
sists of a zinc and a platinized silver plate dipping into 
dilute sulphuric acid; the silver plate, having its surface 
thus covered with a rough coat of finely divided platinum, 
gives up the hydrogen bubbles freely, nevertheless in a bat- 
tery of Smee Cells the current falls off greatly after a few 
minutes. 

(b) Chemical Means. — If a strongly oxidizing substance 
be placed in the solution it will combine with the hydrogen 
and thus will prevent both the increase in internal resistance 



G2 PRACTICAL APPLIED ELECTRICITY 

and the opposing electromotive force. Such substances are 
bichromate of potash, nitric acid, and bleaching powder (so- 
called chloride of lime). These substances, however, would 
attack the copper in the zinc-copper cell. Hence, they can 
only be used in a zinc-carbon or zinc-platinum cell. 

(c) Electro-chemical Means. — It is possible by employing 
double cells to so arrange matters that some solid metal, such 
as copper, shall be liberated, instead of hydrogen bubbles, 
at the point where the current leaves the liquid. This elec- 
tro-chemical exchange entirely obviates polarization. 

68. Internal Resistance. — The resistance offered by a cell 
to a current through it from one plate to the other is called 
its internal resistance. The value of the internal resistance 
of any cell will depend upon the area of the two plates, the 
distance between the two plates, the specific resistance of 
the liquid, and the degree of polarization. As the polariza- 
tion of a cell increases, the internal resistance increases, 
since the effective area of the plates exposed to the action 
of the liquid is decreased due to the accumulation of the 
hydrogen gas. This increase in internal resistance of a cell 
causes the difference in potential between its terminals to 
decrease, as a larger part of the electromotive force of the 
cell is required to force the current through its own resist- 
ance and the available electrical pressure is decreased. 

69. Factors Determining the Electromotive Force of a Cell. 
— When two plates of the same material, such as zinc, are 
immersed in an acid solution and are connected by a 
wire, there will be no current in the wire, because there is 
a tendency to opposite currents and these two tendencies 
neutralize each other. In other words, the difference in elec- 
trical potential between one zinc plate and the solution is 
the same as that between the other zinc plate and the solu- 
tion, and these two potentials are opposite in direction — when 
the two plates are connected by a conductor — which results in 
no current through the circuit when it is closed. The essential 
parts of any cell, therefore, are two dissimilar materials 
immersed in a solution, one of which is more readily acted 
upon by the solution than the other. The greater this 
difference in intensity in chemical action, the greater the 
difference in potential between the terminals of the cell. 

Copper, platinum, silver, and zinc are the only metals 



PEIMAKY BATTEEIES " 63 

that have been mentioned up to the present time, but other 
metals may be used, and since the intensity of chemical 
action will be different for different metals, there will be 
combinations that will produce better results than others. 
For example, a cell composed of zinc and tin would not 
produce as large an electromotive force as one composed 
of zinc and copper the same size, because there is a greater 
difference of electrical potential between the zinc and the 
copper than there is between the zinc and the tin. 

The solution used in the cell also determines the value 
of the difference in potential between any combination of 
plates. There will be a different value for the potential 
difference between any two plates when they are immersed 
in different liquids. 

When the same kinds of metals and solution are used, the 
potential difference between the plates will be the same, 
regardless of the areas of the plates. A small battery will 
have thie same electromotive force as a large one composed 
of the same materials. 

In the following list, the substances are arranged in order 
depending upon the degree of chemical action when placed 
in dilute sulphuric acid: Zinc, Iron, Tin, Lead, Copper, 
Silver, Platinum, and Carbon. 

70. Classification of Cells. — (a) If a cell is capable of 
producing a current directly from the consumption in it of 
some substance, such as zinc, it is a primary cell. If, how- 
ever, a current must first be sent through the cell to bring 
it to such a condition that it is capable of producing a cur- 
rent, it is called a secondary, or storage, cell. The funda- 
mental distinction then between a primary and a secondary, 
or storage, cell is that, with the latter type the chemical 
changes are reversible, while with the former type this is not 
practical even when possible. The discussion of the storage 
cell will be taken up in a later chapter. 

(b) Cells are also classified into closed- and open-circuit 
types, depending upon whether they are or are not capable 
of furnishing a current continuously. This classification is 
entirely dependent upon the polarization — the cell which 
does not polarize being able to maintain its current until 
its chemical substances are exhausted. 

The Grenet and the Leclanche are perhaps the best exam- 



64 



PRACTICAL APPLIED ELECTRICITY 



i:ies of the open-circuit cells, while the Daniell, the Lalande, 
and the Fuller are good examples of the closed-circuit cell. 

(c) All cells must be made up of two substances im- 
mersed in a liquid, but in some cases there are different 
liquids separated by gravity or a porous cup. Cells may then 
be classified, as to their construction, into single-fluid cells 
and double-fluid cells. The Grenet, Leclanche, and Lalande 
are good examples of single-fluid cells, while the Bunsen, 
Fuller, Daniell, and Grove are perhaps the best examples 
of the double-fluid type. 

71. Forms of Primary Cells. — The various cells given in 
Table No. V are the principal ones that are used to any 
extent acd the construction of a few of these will be given in 
detail in the following sections. 

TABLE NO. V 
PRIMARY CELLS 



Names of 
Ceil 


Neeative 
Pole 


Positive 
Pole 


Solution 


Depolariz- 
ing Agent 


E.M.F. 

in 
Volts 


Internal 
Resistance 
in Ohms 


Smee 


Zinc 


Platinized 
Silver 


Solution of 

Sulphuric 

Acid 


None 


.65 


0.5 


Grenet 


Zinc 


Graphite 
(Carbon) 


Solution of 

Sulphuric 

Acid 


Potassium 
Bichro- 
mate 


2.1 


2 to .5 


Leclanche 


Zinc 


Graphite 
(Carbon) 


Ammoni'm 
Chloride 


Manganese; .5 to 
Dioxide ; L6 


1.5 


Daniell.... 


Zinc 


Copper 


Zinc Sul- 
phate 


Copper 
Sulphate 


1.079 


2 to 6 


Lalande... 


Zinc 


Graphite 
(Carbon) 


Caustic 
Potash or 
Potassium 
Hydrate 


Cupric 
Oxide 


0.8 to 
0.9 


1.3 


Fuller 


Zinc 


Graphite 
(Carbon) 


Sulphuric 
Acid 


Potassium 
Bichro- 
mate 


2.0 


0.5 to 0.7 


Bunsen 


Zinc 


Graphite 
(Carbon) 


Dilute 

Sulphuric 

Acid 


Nitric 
Acid 


1.8 to 
1.98 


.08 to. 11 


Grove 

riark 
Standard.. 


Zinc 
Zinc 


Platinum 
Mercury 


Dilute 

Sulphuric 

Acid 

Zinc 
Sulphate 


Nitric 
Acid 

Mercurous 
Sulphate 


1.96 
L434 


.lto.l2 
.3 to .5 


Weston 


Cadmium 


Mercury 


Cadmium 
Sulphate 


Mercurous 
Sulphate 


l.OlScC 





il 



PEIMARY BATTERIES 65 

72. Chemicals Used In Cells and Their Symbols. — 

Sulphuric Acid, H2SO4. 

Chromic Acid, CrOs. 

Manganese Dioxide, MnO. 

Zinc Chloride, ZnCl2. 

Lead Oxide, PbO. 

Zinc Sulphate, ZnS04. 

Nitric Acid, HNO3. 

Hydrochloric Acid, HCl. 

Silver Chloride, AgCl. 

Copper Oxide, CuO. 

Lead Peroxide, Pb02. 

Sodium Chloride, NaCl. 

Caustic Potash or Potassium Hydrate, KOH. 

Copper Sulphate (blue vitriol), CuSO^. 

Zinc Sulphate (white vitriol), ZnS04. 

Ammonium Chloride (sal-ammoniac), NH4CI. 

Bichromate of Potassium, K2Cr207. 

Bichromate of Soda, Na2Cr207. 

Mercurous Sulphate, Hg2S04. 

Cadmium Sulphate, CdS04. 

73. Mechanical Depolarization. — The Smee cell has been 
mentioned in section (67) as a good example of a mechanical 
means of preventing polarization. This cell was used com- 
mercially a number of years ago, but it was not very suc- 
cessful. A Smee cell is shown in Fig. 24. 

There are a number of cells used for intermittent work, 
such as ringing door bells, that depend entirely upon the 
use of a large positive plate surface to lessen the rapidity 
of polarization. They consist usually of zinc and carbon 
plates immersed in a solution of sal ammoniac (ammonium 
chloride), see section (72).. The carbons for such cells are 
made in almost endless variety and with very large surface. 

74. Chemical Depolarization. — Bichromate Cells. — There 
are a number of different forms of bichromate cells, the 
principal ones of which are perhaps the Grenet and Fuller. 

(a) Grenet Cell. — In this form a zinc plate is suspended 
by a rod between two carbon plates, see Fig. 25, so that 
it does not touch them, and when the cell is not in use the 
zinc can be removed from the solution by raising and fasten- 



66 



PEACTICAL APPLIED ELECTRICITY 



II 



ing the rod by means of a set screw, as the acid acts on 
the zinc when the cell is not in use. Sulphuric acid and 
water is the solution used in this cell, to which is added 
potassium bichromate that acts as the depolarizer. The 
bichromate is rich in oxygen, which readily combines with 
the liberated hydrogen and thus prevents polarization. This 





Fig. 24 



Fig. 25 



cell gives a large e.m.f. and 'is capable of supplying a 
strong current for a short time, but the liquid soon becomes 
exhausted. 

(b) Fuller Cell. — This form of bichromate cell is a double 
fluid type, and has the advantage over the Grenet type in 
that the zinc is always kept well amalgamated and it is not 
necessary to remove it from the solution. A pyramidal block 
of zinc is placed in a small porous cup, Fig. 26, and a 
small quantity of mercury poured in. The cup is then filled 
with a diluted solution of sulphuric acid and placed in a 
glass jar containing a solution of potassium bichromate 
and the carbon plate (P). A conductor, covered with a suit- 
able insulation, is attached to the block of zinc and serves as 
one terminal of the cell. The zinc is well amalgamated by 
the mercury and there is practically no local action. The 



PRIMARY BATTERIES 



67 



cell gives a large e.m.f. and may be used for open circuit 
or semi-closed circuit work. 

75. Chemical Depolarization. — Leclanche. — This cell con- 
sists of a zinc plate in a solution of ammonium chloride and 
a carbon plate placed inside a porous cup which is packed 
full of manganese dioxide and powdered carbon. The action 
of the manganese dioxide on the hydrogen is not quick 
enough to prevent polarization entirely when large currents 




Zinc 




Pig. 26 



Fig. 27 



are taken from the cell. The cell, however, will recover 
when allowed to stand on open circuit. A great advantage of 
this type of cell lies in the fact that the zinc is not acted 
on at all by the ammonium chloride when the cell is on 
open circuit, and as a result it can be left for almost an 
indefinite period when the circuit is open without deteriora- 
tion. These cells are usually used for intermittent work, 
such as ringing door bells, and will supply quite a large 
current for a short time. Their e.m.f. is about 1.5 volts. 
Leclanche cells are called open-circuit cells on account of 
the very slight chemical action that takes place when the 
circuit is open. A Leclanche cell is shown in Fig. 27. 

76. Electro-Chemical Depolarization. — Daniell Cells. — This 
type of cell consists of a zinc plate immersed in a solu- 
tion of zinc sulphate and a copper plate Immersed ui a 



68 



PEACTICAL APPLIED ELECTRICITY 



solution of copper sulphate. The two liquids may be kept 
apart either by gravity or by a porous earthen cup, as shown 
in Fig. 28. When the solutions are kept separated by grav- 
ity, the cell is called the gravity, or crowfoot type. A cross- 
section of a Daniell cell, in which the liquids are separated 
by a porous cup, is shown in Fig. 29. The gravity cell is 




Fig. 28 



Cu 



/CaS 



ZnSo 



Zn. 



n 



Fig. 29 



shown in Fig. 30. The copper sulphate being the heavier 
of the two liquids remains at the bottom about the plate 
of copper, while the zinc sulphate remains at the top about 
the zinc plate. This cell will give a very constant e.m.f. of 
about 1.08 volts. It has a large internal resistance (two 
to six ohms) and as a result is not capable of supplying 
a very large current. The current supplied, however, is con- 
stant, and it will operate for a great length of time without 
renewal. 

The Daniell cell is a closed-circuit cell and it should 
never be allowed to stand on open circuit, but a resistance 
(thirty to fifty ohms) should always be connected across 
its terminals. 

77. Dry Cells. — The dry cell is a special form of the 
Leclanche cell first described. The cell is not altogether 
dry, since the zinc and carbon plates are placed in a moist 
paste which consists usually of ammonium chloride, one 
part; plaster of Paris, three parts; zinc chloride, one part; 



PRIMARY BATTERIES 



69 



zinc oxide, one part, and sawdust. These various materials 
' composing the above mixture are thoroughly mixed and then 
moistened with a small quantity of water. The paste thus 
formed is packed around a carbon rod, placed inside a zinc 
^cup lined with moistened blotting paper, and the cup is sealed 
■ with some kind of wax to prevent evaporation. There are a 
large number of different makes of dry cells on the market 
at the present time but the chemical action in each is prac- 
tically the same. The dry cell is a very convenient form 
of cell and its operation is very satisfactory for work requir- 
ing an intermittent current. Fig. 31 shows a cross-section 
through a dry cell. 








<KPa5te Board 
Covering 

r- Pitch 
-Zinc Cup 
-Filling 
- Carbon 
-Blotting 
Paper 



Fig. 30 



Fig. 31 



78. Standard Cells. — A standard cell is one whose e.m.f. 
can be accurately calculated and will remain constant. The 
Clark Standard cell and the Weston cell are the two best 
examples of standard cells. The Clark cell has an e.m.f. of 
1.434 volts at 15°C. and a correction must be made in this 
value when the cell is used at some other temperature. 

The Weston normal cell has an e.m.f. of 1.01830 volts and 
there is practically no change in its e.m.f. due to a change 
in temperature. A Weston standard cell, as manufactured by 
the Weston Electrical Instrument Company, is shown in 
Fig. 32. The e.m.f. of different Weston cells is not exactly 
the same, but they are standardized in the factory and a 
certificate accompanies each cell. The average e.m.f. of a 



70 



PBACTICAL APPLIED ELECTEICITY 




Fig. 32 



number of cells tested by the Bureau of Standards at Wash- 
ington, D. C, was 1.01869 volts. 

A great amount of care is exercised in assembling standard 
cells, to see that the materials used are the very best and 
that they are all constructed alike. 

79. Requirements of Good Cell. 
— A good cell should fulfill all, or 
the greater part, of the following 
conditions: 

(a) Its electromotive force 
should be high and constant. 

(b) Its internal resistance should 
be small. 

(c) It should be capable of sup- 
plying a constant current, and, 
therefore, entirely free from polari- 
zation, and not liable to rapid 
exhaustion, requiring frequent re- 
newals of the liquid or plates. 

(d) It should .be free from local action. 

(e) It should be cheap and of durable materials which 
results in a low cost for renewals when they are required. 

(f) It should be easily managed, and, if possible, should 
not emit corrosive fumes. 

No particular cell fulfills all of the above conditions, how- 
ever, and some cells are better for one purpose than others. 
Thus, for telegraph work over a long line, a cell of consid- 
, erable internal resistance is no great disadvantage; while a 
large internal resistance is a great disadvantage when the 
cell is supplying current to a circuit of corresponding low 
resistance. An open-circuit cell would not operate satisfac- 
torily in driving a small motor, but would be quite satisfac- 
tory for intermittent work; while the closed-circuit type 
would be very unsatisfactory for intermittent work. 

80. Series Connection of Cells. — Any number of cells are 
said to be connected in series when the positive terminal 
of one is connected to the negative terminal of another, 
and so on. When all of the cells are connected, there remains 
a positive and a negative terminal which form the terminals 
of the battery. If (n) cells are connected in series, as shown 
in Fig. 33, and they each have an e.m.f. of (e) volts, the 



PEIMAKY BATTERIES 



71 



combination will have an e.m.f. of (ne) volts. The internal 
resistance of the battery will be equal to (nr) ohms where 
(r) is the internal resistance of each cell. If the external 
resistance or the resistance of the circuit to which the battery 
is connected is (R) ohms, then the current produced by the 
combination will be 

ne 

I = (70) 

R + nr 

In the above equation (R + nr) represents the total resist- 
ance of the entire circuit and (ne) the total electromotive 



f -r T 



f— iHh'— 1 

y — A/www — ^ 



Fig. 33 




force acting in the circuit. Fig. 83 sho^\rf three cells con- 
nected in series and the battery thus formed connected to 
a resistance (R). 

Fig. 34 shows a hydraulic analogy similar to the connection 
of cells in series. The pumps (Pi) and (P2) are so arranged 
that their pressures are added, the total pressure acting in 
the circuit being equal to the sum of the pressures produced 
by the respective pumps. If there is no water flowing and 
the gauge (Gi) reads zero, then (G2) reads the pressure 
produced by the pump (Pi). The difference in the readings 
of (G3) and (G2) is the pressure produced by the pump 
(P2). Hence (G3) reads the total pressure produced by the 
two pumps combined when (Gi) reads zero and no water is 
fio^wing. When there is a flow of water, however, part of the 
pressure produced by the pumps is used in causing the water 
to pass through them or to overcome their internal resistance, 
and as a result the indication on (Go) is reduced. 

Example. — Six cells having an e.m.f. of 1.5 volts each and 
an internal resistance of .6 ohm each are connected in series 



72 



PEACTICAL APPLIED ELECTRICITY 



with a resistance of 10 ohms. What is the current in 
the resistance when the circuit is closed? 

Solution. — By a direct substitution in equation (70) we 
have 



G X 1.5 



6.62— 



10 + (6 X .6) 10 + 3.6 



13.6 

Ans. 6.62 amperes. 



81. Parallel Connection of Cells. — When (n) cells, all 
having the same e.m.f., are all connected in parallel, then 
the e.m.f. of the combination is only that of a single cell. 
The internal resistance of a battery formed of a number of 
cells connected in parallel is less than the internal resistance 




b 



y AAAA/VW — ^ 



Fig. 35 




Fig. 36 



of a single cell. If (t) is the internal resistance of each cell, 
and there are (n) cells in parallel, then the total internal 



resistance will be 



(—). If the resistance of the external 

circuit is (R) ohms, then the current produced by the com- 
bination will be 



1 = 



In the above equation 



r 
R + — 

n 

(«+n) ' 



(71) 



is the total resistance of 



PEIMAEY BATTERIES 73 

the entire circuit, and (e) is the electromotive force acting 
on this resistance. 

Fig. 35 shows three cells connected in parallel and the 
battery thus formed connected to a resistance (R). 

Fig. 36 shows a hydraulic analogy similar to the con- 
nection of cells in parallel. The pumps (Pi) and (P2) ars 
so connected that their pressures are not added, but the 
quantity of water supplied will be equal to that supplied 
by both pumps. 

Example. — Six cells having an e.m.f. of 1.5 volts each and 
an internal resistance of .6 ohm each are connected in 
parallel, and the combination is then connected to a resist- 
ance of 10 ohms. What current exists in the resistance when 
the circuit is closed? 

Solution. — By a direct substitution in equation (71) we 
have 

1.5 " 1.5 

I = = = 0.148+ 

.6 10.1 
10 + — 



6. 



Ans. 0.148-{- ampere. 



82. Series and Parallel Combinations.-^A very common 
grouping of cells is a combination of the series and the par- 
allel groups. Suppose there are (P) groups of cells and each 
group consists of (S) cells in series. The total number of 
cells is then equal to (SP). The e.m.f. will be (Se) volts, 
the internal resistance of each set will be (Sr) ohms, and 
the internal resistance of the (P) sets combined will be 

/Sr\ 

(-pr) ohms. If the external resistance is (R) ohms, then 

the total resistance will be ( R4- p ) ohms, and the current 
produced by the combination will be 

Se PSe 

1=-- ^ (72) 

Sr PR + Sr 
R + - 
P 



w 



PBACTICAL APPLIED ELECTKICITY 



Fig. 37 shows a battery composed of three groups of cells 
and there are three cells connected in series in each group. 
The battery is connected to a resistance (R). 

Example. — A battery is composed of nine cells connected, 
as shown in Fig. 37, to an external resistance of 10 ohms. 
The e.m.f. of each cell is 1.5 volts; each cell has an internal 
resistance of .5 ohm. What current will the battery supply 
when the circuit is closed? 

Solution. — Substituting directly in equation (72), we have 



3 X 3 X 1.5 



13.5 



: 0.428 + 



(3 X 10) + (3 X .5) 



31.5 

Ans. 0.428+ ampere. 
83. Advantage of Series and Par- 
allel Connections. — Cells are con- 
nected in parallel when it is desired 
to obtain a large current through 
a low external resistance. When the 
cells are so grouped they are equiva- 
lent to one large cell, and will have 
a very low internal resistance, and 
when connected to a low external 
resistance, as compared to the inter- 
nal resistance, the current will be 
large. If the external resistance 
is large, the current will be small, as the electromotive 
force acting is small. The series connection is employed 
when the external resistance is the principal resistance to 
overcome and the maximum current strength is desired in 
the circuit. 






R 

^ — AAA/WW — ^ 

Fig. 37 



PROBLEMS ON GROUPING OF CELLS 



(1) How many cells should be connected in series to 
cause a current of 1.5 amperes in an external resistance of 
8 ohms. The e.m.f. of each cell is 1.5 volts, and its internal 
resistance is .2 ohm. 

Ans. 10 cells. 

(2) How many cells must be connected in parallel to 
cause a current of 2 amperes in an external resistance of .6 



PRIMAEY BATTERIES 75 

ohm. The e.m.f. of each cell is 1.5 volts and its internal 
resistance is 1.2 ohms. 

Ans. 8 cells. 

(3) Ten cells, each having an e.m.f. of 2.2 volts and an 
internal resistance of .07 ohm, are connected in series to a cir- 
cuit whose resistance is 4.3 ohms. What current exists in the 
circuit? 

Ans. 4.4 amperes. 

(4) Twenty storage cells are connected in series-multiple, 
there being five groups in parallel and four cells in series 
in each group. The e.m.f. of each cell is 2.2 volts and its 
internal resistance is .05 ohm. What current will the com- 
bination produce through an external resistance of .4 ohm? 

Ans. 20 amperes. 

(5) Five cells are connected in series. Each cell has an 
e.m.f. of 2 volts and an internal resistance of .2 ohm. What 
is the terminal voltage of the battery on open circuit and 
also when it is connected to an external resistance of 4 
ohms? 

Ans. Open-circuit voltage == 10 volts. 
Closed-circuit voltage = 8 volts. 

Note. — (There will be a drop in terminal voltage when the 
circuit is closed on account of the internal resistance.) 

(6) Twenty cells, each having an e.m.f. of 2 volts and an 
internal resistance of .2 ohm, are to be connected so that 
they will give the maximum current through an external 
resistance of 1 ohm. What combination should be used? 
Ans. Two groups, in parallel, each group having ten cells 

in series. 

Note. — (The maximum current is obtained from any com- 
bination of cells when they are so connected that their 
combined internal resistance is equal to the external resist- 
ance to which they are connected). 

Solution. — Let (S) equal the number of cells in series in 
any one group and then (n -^^ S) will equal the number of 
groups in parallel. Then in order that the maximum cur- 
rent be obtained from the battery 



76 



or 



PEACTICAL APPLIED ELECTEICITY 

S X r 

must equal 1.0 

(n-^S) 



S X .2 .2S2 2S2 S2 



= 1.0 



20 



20 200 100 



S2 = 100 
S= 10 

(7) How many cells, each having an e.m.f. of 2.2 volts 
and an internal resistance of .005 ohm, must be connected 
in series in ord^r that the terminal voltage may be at least 
44 volts when the current through the battery is 40 amperes? 

Ans. 22 cells. 

(8) The terminal voltage of a battery drops from 1.8 
volts on open circuit to 1.5 volts when there is a current 
of 6 amperes through the battery. What is the internal 
resistance of the battery? 

Ans. .5 ohm. 



^ 



CHAPTEK V 

MAGNETISM 

84. The Magnet. — The name magnet was given by the 
ancients to certain black stones, found in various parts of 
the world, principally at Magnesia in Asia Minor, which 
possessed the property of attracting to them small pieces 
of iron or steel. This magic property, as they deemed it, 
made the magnet-stone famous; but it was not until about 
the twelfth century that such stones were discovered to have 
the still more remarkable property of pointing approximately 
north and south when freely suspended by a thread. This 
property of the magnet-stone led to its use in navigation, 
and from that time the magnet received the name of **lode- 
stone," or "leading stone." The natural magnet, or lodestone, 
is an ore of iron, and is called magnetite. Its chemical 
composition is Fe304. This is found in quite large quantities 
in Sweden, Spain, and Arkansas, U. S. A., and other parts 
of the world, but not always in the magnetic state. 

85. Artificial Magnets. — If a piece of iron, or, better still, 
a piece of hard steel, be rubbed with a lodestone, it will be 
found to possess the properties or characteristic of the mag- 
net, viz, it will attract light 

bits of iron; it will point ap- 
proximately north and south 
if hung up by a thread; 
and it can be used to magne- 
tize another piece of iron Pig. 33 
or steel. Magnets made in 
this manner are called artificial magnets. 

Strong artificial magnets are not made from lodestone, 
as its magnetic force is not strong, but by methods as de- 
scribed under **Electromagnetism." Figs. 38 and 39 show, 
respectively, a natural and an artificial magnet, each of which 

77 




78 



PEACTICAL APPLIED ELECTKICITY 










Fig. 39 



has been dipped into iron filings; the filings are attracted 
and adhere in tufts at the ends. 

86. Poles of a Magnet. — Certain parts of a magnet possess 
the property of attracting iron to a greater extent than do 
other parts. These parts are called the poles of the magnet. 

The poles of a bar magnet, 
for example, are usually 
situated at or near the 
ends of the bar, as shown 
in Fig. 38. 

87. Magnetic Needle. — 
The magnetic needle con- 
sists of a light needle cut 
out of steel, and fitted 
with a cap of glass, or agate, by means of which it can be 
supported on a sharp point, so as to turn with very little 
friction. This needle is made into a magnet by being rubbed 
on a magnet; and when placed on its support will turn into 
the north-and-south position, or, as we should say, will set 
itself in the "magnetic meridian." The end of the needle 
that points toward the north geographical pole is called the 
North Pole, and is usually marked with the letter N, while 
the other end is the South Pole. 

By the term polarity is 
meant the nature of the 
magnetism at some particu- 
lar point, that is, whether it 
is north or south-seeking 
magnetism. The compass 
sold by opticians consists of 
such a needle balanced 
above a card marked with 
the "points of the compass" 

and the whole placed in a suitable containing case, 
mon form of the magnetic needle is shown in Fig. 40. 

88. Magnetic Attrac-ion and Repulsion. — When the two 
poles of a magnet are presented in turn to the north-pointing 
pole of a magnetic needle, it will be observed that one pole 
of the magnet attracts it, while the other repels it. If the 
magnet is presented to the south-pointing pole of the mag- 
netic needle, it will be repelled by one pole and attracted 




Fig. 40 



A com- 



MAGNETISM 79 

by the other. The same pole that attracts the north-pointing 
end of the magnetic needle repels the south-pointing end. 
As the needle and the magnet attract each other when unlike 
poles are presented, and repel each other when like poles 
are presented, it follows that like poles always repel each 
other and unlike poles always attract each other. Fig. 41 
shows the results of presenting two like poles and Fig. 42 
shows the result of presenting two unlike poles. 






Fig. 41 Fig. 42 

Two equal and like poles are said to have unit strength 
when there is a force of repulsion between them of one 
dyne when placed one centimeter apart in air. 

89. Magnetizable Metals. — The principal magnetic metals 
used in practice are steel and iron. There are other metals, 
such as nickel, cobalt, chromium, and cerium, that are at- 
tracted by a magnet, but very feebly. Of this last class 
cobalt and nickel are the best, but very inferior to iron or 
steel. All other substances, such as wood, lead, gold, copper, 
glass, platinum, etc., may be regarded as unmagnetizable, 
or nonmagnetic substances. Magnetic attraction or repul- 
sion will, however, take place through these substances. 

90. Magnetic Force. — The force with which a magnet 
attracts or repels another magnet, or any piece of iron or 
steel, is termed its magnetic force. The value of this mag- 
netic force is not the same for all distances, the value being 
greater when the magnet is nearer, and less when the magnet 
is further off. The value of this force of attraction or repul- 
sion decreases inversely as the square of the distance from 
the pole of the magnet. The force is mutual, that is, the 



80 



PRACTICAL APPLIED ELECTRICITY 



iron attracts the magnet just as much as the magnet attracts 
the iron. 

91. Magnetic Lines of Force. — The magnetic force pro- 
duced by a magret emanates in all directions from the mag- 
net. The direction of the magnetic force at any point 
near a magnet can be determined by means of a small 
compass needle suspended at the point in such a way that it 
is free to move in any direction. The direction at any other 
point can be determined by changing the position of the 
needle with respect to the magnet. Starting with the needle 
near one end of the magnet, it may be carried toward the 
other end and an imaginary line, drawn in such a way that 
its direction at any point corresponds to the direction as- 
sumed by the needle at that point, corresponds to what is 
termed a line of force, or it is the path taken by a north 
magnetic pole in moving from the north to the south pole 
of a magnet. These lines of force start at the north pole 
of a magnet and terminate at the south pole. Fig. 43 shows 
such a line. 

92. Magnetic Field. — The region surrounding a magnet 
which is permeated by magnetic lines of force is palled a 



^HEIZZZUp 






Fig. 43 



Fig. 44 



magnetic field of force, or a magnetic field. The lines of 
force forming a magnetic field emanate from the N-pole 
of a magnet, pass through the medium surrounding the 
magnet, re-enter the S-pole and complete their path by 
passing from the S-pole to the N-Pole inside the magnet 
itself. The magnetic field surrounding a bar magnet is 
shown in Fig. 44. All magnetic lines form closed circuits 
and there must be two or more magnetic poles (always even) 
associated with each of these circuits except in the case of a 
ring magnetized by a current, which will be discussed later. 



MAGNETISM 



81 



The region we speak of as a magnetic field is capable of 
acting upon magnets, magnetic materials, and conductors 
carrying a current of electricity. The lines of force forming 
any magnetic field are assumed to have two properties: 
First, they tend to contract in length; second, they repel 
each other. The attraction and repulsion of unlike and like 
poles can be accounted for by assuming the lines to possess 
the above properties. 

93. Making Magnetic Fields. — A graphical representation 
of a magnetic field may be made by placing a piece of card- 
board over the magnet or magnets whose field you want to 
produce, and sprinkle iron filings on the paper, tapping it 
gently at the same time. The iron filings are composed of 
a magnet material and arrange themselves in the direction 
of the lines of force or magnetic field, and as a result produce 
a graphical representation of the field. This representation 
of the field can be made permanent by using a piece of paper 
that has been dipped in parafiine instead of the cardboard. 







Fig. 45 



Fig. 46 



The parafiine can be heated by means of a warm soldering 
iron, or other warm non-magnetic material, which permits 
the filings to imbed themselves in the wax and they will be 
lield firmly in place when the parafiine has cooled. 

The magnetic field that exists between unlike poles is 
shown in Fig. 45, and the field between like poles is shown 
in Fig. 46. 

94. Distortion of Magnetic Field. — The direction of a mag- 
Qetic field is infiuenced by the presence of magnetic material 
or magnets. When a magnetic material is placed in any 
magnetic field that exists in air, the form of the field will be 
changed because the material is a better conductor of mag- 
aetic lines than air and the lines of force crowd into the 
naterial. There will be a greater number of magnetic lines 



82 



PKACTICAL APPLIED ELECTRICITY 



in a given area in the iron than there is in a corresponding 
area in the air. A magnetic field that has been distorted, 
due to the presence of a piece of iron, is shown in Fig. 47. 
All materials that conduct magnetic lines better than air 
are called paramagnetic, and those that do not conduct as 
well as air are called diamagnetic substances. 





Fig. 47 



Fig. 48 



95. Magnetic Induction. — Magnetism may be communicated 
to a piece of iron without actual contact with the magnet. 
If a short, thin, unmagnetized bar of iron be placed near 
some filings, and a magnet brought near to the bar, the 
presence of the magnet will induce magnetism in the bar, and 
it will now attract the iron filings, Fig. 48. The piece of 
iron thus magnetized has two poles, the pole nearest to the 
pole of the inducing magnet being of the opposite kind, while 
the pole at the farther end of the bar is of the same kind as 
the inducing pole. Magnetism can, however, only be induced 
in those bodies that are composed of magnetic materials. 
It is now apparent why a magnet should attract a piece of 
iron that has never been magnetized; it first magnetizes it 
by induction and then attracts it; as the nearest end will 
be a pole of opposite polarity, it will be attracted with 
a force exceeding that with which the more distant end is 
repelled. 

96. Retention of Magnetization. — Not all of the magnetic 
substances can be used in making permanent magnets, as 
some of them do not retain their magnetism after being mag- 
netized. The lodestone, steel, and nickel, retain permanently 
the greater part of the magnetism imparted to them. Cast 
iron and many impure qualities of wrought iron also retain 
magnetism imperfectly. Pure, soft iron is, however, only 



MAGNETISM 83 

temporarily magnetic. The above statements can be illus- 
trated by the following experiment: Take several pieces 
of soft iron, or a few soft iron nails, and place one of them 
in contact with the pole of a permanent magnet, allowing it 
to hang downward from the magnet, as shown in Fig. 49. 
The piece of iron or nail is held to the magnet because it 
has become a temporary magnet, due to the process of mag- 
netic induction. Another piece can be hung to the first, and 
another to the second, etc., until a chain of four or five 
pieces is formed. If now the steel magnet be removed from 
the first piece of iron, or nail, all the remaining pieces drop 
off and are no longer magnets. A similar chain formed of 
steel needles will act in the same way, but they will retain 
their magnetism permanently. 

It is harder to get the magnetism into steel than into 
iron, and it is harder to get the magnetism out of steel than 
out of iron, because steel resists magnetization or demagneti- 
zation to a greater extent than soft iron. This power of resist- 
ing magnetization or demagnetization is called hysteresis. 

97. Molecular Theory of Magnetism. — There are quite a 
number of experimental facts that lead to the conclusion that 
magnetism has something to do with the molecules of the 



^ 



^^ 




Pig. 49 

substance, since any disturbance of the molecules causes a 
change in the degree of magnetization. If a test tube full 
of hard steel filings be magnetized, it will behave toward a 
compass needle or other magnet as though it were a solid 
bar magnet, but it will lose practically all of its magnet- 
ism as soon as the fillings are rearranged with respect to 
each other by giving the tube several good shakes. A 
needle that has been magnetized will lose its magnetism 
when heated. A magnet may be broken into any number of 



84. 



PRACTICAL APPLIED ELECTRICITY 



I 






Fig. 51 



different pieces and there will appear at each break an N- 
pole and an S-pole, as shown in Fig. 50. The strength of 
the poles of any magnet will be greatly reduced by ham- 
mering, twisting, or bending it. A theory often used to explain 
certain magnetic phenomena is as follows: In an unmagne- 
tized bar it is assumed that the molecules are each a tiny 

magnet, and that these 
molecules or magnets are 
arranged in no definite way, 
except that the opposite 
poles neutralize each ether 
throughout the bar. The 
theoretical arrangement of 
the molecules in an unmagnetized bar is shown in Fig. 
51. When the bar is brought into a magnetic field, the tiny 
magnets are turned, due to the action of the outside force, 
so that the N-poles tend to point in one direction and their 
S-poles in the other. The arrangement of the molecules after 
the bar has been magnetized is shown in Fig. 52. The 
opposite poles neutralize each other in the middle of the bar 
but there will be an N-pole found at one end and an S-pole 
at the other. 

'Ehe ease with which any ^| 

material may be magnetized 
as compared to some other 
material will depend upon 
what might be termed the 
molecular friction of the ma- 
terial. Thus, the molecules in " 
a bar of steel offer a greater resistance to a change in their 
position than do the molecules in cast iron. Steel, as a result, 
is harder to magnetize than cast iron, and it will also retain 
its magnetism after once magnetized better than cast iron for 
the same reason. 

98. Application of Permanent Magnets. — Permanent mag- 
nets are made to assume many different forms, depending 
upon the particular use to which they are to be placed. In 
the majority of cases the bar forming the magnet is bent 
into such a form that both poles will produce an effect 
instead of only one pole. Thus, instead of using a straight bar 
magnet in picking up a piece of iron, as shown in Fig. 53, the 









Fig. 52 



MAGNETISM 



85 



bar can be bent into a U-shape and both poles presented to 
the piece to be picked up as shown in Fig. 54. The effect 
of the two poles in the second case will, of course, be greater 



Bl 




Fig. 53 



Fig. 54 



than the single pole in the first. A magnet such as that shown 
in Fig. 54 is called a horseshoe magnet. 

Permanent magnets are used for numerous different pur- 
poses, such as in telephone receivers, relays, ringers, measur- 
ing instruments, etc. 



CHAPTEK VI 

ELECTROMAGNETISM 

99. Magnet Field Around a Conductor Carrying a Current.— 
In 1819, Oersted discovered that a magnetic needle was dis- 
turbed by the presence of a conductor carrying a current, 
and that the needle always tended to set itself at right an- 
gles to the conductor. If a magnetic needle be placed below 






a wire, as shown in Fig. 55, the current in the wire being 
from left to right, as indicated by the arrow, the needle 
will tend to move in the direction indicated by the curved 
arrows. If the current in the conductor be reversed, the 
direction of the magnetic needle will be reversed. It is thus 
seen that there is a magnetic field set up about a conductor 
carrying a current and that the direction of this magnetic 
field will depend upon the direction of the current in the 
conductor. Magnetism set up in this way by an electric 
current is called electromagnetism. 

100. Direction of an Electromagnetic Field. — Remembering 
that the direction of a magnet field may be determined 
by placing a compass needle in the field and determining 
the direction in which the N-pole of the needle will point — 
this being taken as the positive direction — you can determine 
the direction of the magnetic field surrounding a conductor, 
produced by a current in the conductor. The small circle 

86 



4 



ELECTKOMAGNETISM 



87 



in Fig. 56 represents the cross-section of a conductor that 
can be imagined as passing through the paper. The direc- 
tion of current in this conductor is away from the observer, 
and this fact is indicated by the plus sign (-f ) inside the 
circle. A compass needle placed below this conductor will 
set itself in such a position that the N-pole is toward the 
left and the S-pole is toward the right. If the compass needle 
be placed above the conductor, as shown in Fig. 57, the N- 
pole will point toward the right and the S-pole toward the 



e 



N<- 



-^N 



Fig. 56 



© 



Fig. 57 



left. When the current is reversed in direction, that is, the 

, flow is toward the observer — which is indicated by the minus 

sign ( — ) inside of the circle — the positions assumed by the 

compass needle will be just the reverse of those shown in 

I Figs. 56 and 57. 




Fig. 58 



If a conductor is passed through a small opening in the 
center of a piece of cardboard that is supported in a hori- 
zontal position, as shown in Fig. 58, and a current is passed 
through the conductor, the field may be explored by means 
of a small compass needle. When the current in the con- 



88 



PBACTICAL APPLIED ELECTRICITY 









ASV'v ^ ->^- 



ductor is down through the cardboard, as indicated by thi 
arrow (I) in the figure, the needle will assume a position 
at right angles to the conductor (neglecting the effect of 
the earth's magnetic field) and the N-pole will point, when 
you are looking down upon the cardboard, in the direction 
the hands of a clock move. The dotted line drawn on the 
surface of the cardboard indicates the path that an N-pole 

would move in, in passing 
around the conductor. The 
arrow on the dotted line 
indicates the direction in 
which the pole would 
move. If the current in 
the conductor were re- 
versed, the direction of 
motion, or the direction of 
the field, would be reversed. 
Iron filings may be 
sprinkled on the cardboard 
and they will form con- 
centric circles about the 
conductor which corre- 
spond to the lines of magnetic force produced by the cur- 
rent. A field formed in this way is shown in Fig. 59. 

101. Rules for Determining the Direction of a Field About 
a Conductor Carrying a Current. — The*e ar©, a number of 
different ways of remembering the relation; b^etween the di- 
rection of a magnetic field and the direct/on of the cur- 
rent producing it. A very simple rule that is known as 
the "right-hand rule" is as follows: Grasp the conductor 
carrying the current with the right hand, the thumb being 
placed along the wire and the fingers being wrapped around 
the wire; then the fingers point in the direction of the 
magnetic field produced by the current in the wire when 
the thumb points in the direction in which the current passes 
through the wire. 

If a person looks along a conductor, carrying a current, 
in the direction of the current, the direction of the magnetic 
field surrounding the conductor will be clockwise. 

Another rule known as the ''right-hand screw rule" is as 
follows: Consider a right-handed screw which is being 



Fig. 59 



ELECTROMAGNETISM 



89 



screwed into or out of a block, as shown in Fig. 60. If 
an electric current is supposed to exist through the screw 
in the direction in which the screw moves through the block, 
then the direction of the magnetic field will correspond to 
the direction in which the screw turns. 

102. Strength of Magnetic Field. — The strength of any- 
magnetic field is measured in terms of the number of lines 
of force per unit area, usually one square centimeter per- 
pendicular to the direction of the field. The symbol used 
to indicate field strength is the letter (E). The properties 
of the magnetic field surrounding a conductor carrying a 
current are the same as those possessed by the magnetic 
field produced by a permanent magnet. The strength of a 







l_s 



Fig. 60 



Fig. 61 



magnetic field at a certain point, due to a current in a con- 
ductor or a permanent magnet, will depend directly upon 
the strength of the current in the conductor, or the strength 
of the permanent magnet, and inversely as the square of the 
average distance the point is from the conductor or perma- 
nent magnet. 

103. Solenoid. — A little consideration will show that if a 
current be carried below a compass needle in one direction, 
and then back in the opposite direction above the needle 
by bending the wire around, as shown in Fig. 61> the forces 
exerted on the needle, due to the current in the upper and 
lower portions of the wire, will be in the same direction. 
If the needle is the same distance from each portion of the 
circuit the effect of the two parts will be just double that 



90 



PRACTICAL APPLIED ELECTRICITY 



3 wire^ 



V 



■'>^. 



1 / }- / 



Fis. G2 



produced by either part acting alone. Hence, if the 
be coiled about the needle, each additional turn will produce 
an additional force tending to turn the needle from its nor-' 
mal position. The magnetic effect of any current can be 
greatly increased in this way. 

A cross-section through a single turn of wire is shown in 

Fig. 62. The current is 

away from the observer in 

/' / /' /'""^^^ '« ^^ '• the upper part of the con- 

ductor, and toward the 
observer in the lower part, 
as indicated by the ( + ) 
and ( — ) signs. The direc- 
tion of the magnetic field 
surrounding the upper 
cross-section will be clock- 
wise, as indicated by the 
arrows on the curves 
drawn about it, while .the 
direction of the field surrounding the lower cross-section will 
be counter clockwise, as indicated by the arrows, since the 
current is in the opposite direction in the lower cross-section 
of the conductor to what 
it is in the upper cross- 
section. It will be seen 
that the magnet field be- 
tween the two cross-sec- 
tions of the conductor 
or through the center of 
the turn is toward the left 
and it is the resultant of 
the two fields about the 
two cross-sections. The 
field is stronger between 
the two cross-sections than 
it is outside, which is indicated by a larger number of lines 
of force per unit of area, as shown in the figure. 

Increasing the number of turns forming the coil will in- 
crease the strength of the magnetic field inside the coil, 
since the lines of force that surround each turn seem to 
join together and pass around the entire winding instead 




Fi- 63 



ELECTROMAGNETISM 



91 



-of passing arourd the respective conductors. A cross-sec- 
tion through a coil composed of several turns is shown in 
Fig. 63. A few of the lines encircle the different conductors, 
but the greater portion pass entirely through the center of 
the coil and around the total number of turns. Such coils 
are called solenoids. 

104. Polarity of Solen- 
oids. — A solenoid carrying 
a current exhibits all the 
magnetic effects that are 
shown by permanent mag- 
nets. If a solenoid that 
is carrying a current be 
suspended so that it is 
free to swing about a ver- 
tical axis, its own axis 
being horizontal, it will 
move into an approxi- 
mately north and south 
position, exactly like a 
permanent magnet. A so- 
lenoid supported in this 
way is shown in Fig. 64. 
They attract and repel 
magnets, pieces of iron, 
and other solenoids. See 
Fig. 65. 

The polarity of any solenoid may be determined, when 
the direction of the current is known, by a simple application 
of any one of the rules given in section (101). The lines of 
magnetic force inside the solenoid pass from the S-pole 
to the N-pole and outside the solenoid from the N-pole to 
the S-pole. Referring to Fig. 63, you see that the end of 
the solenoid toward the left will be the S-pole and the end 
toward the right, the N-pole. 

A simple rule by which the polarity of a solenoid may be 
determined, if the direction of the current around the wind- 
ing is known, is as follows: If you face one end of the 
solenoid and the current is around the winding in a clock- 
wise direction, the end nearest you will be the S-pole and 
the other end will be the N-pole. If the direction of the cur- 




Fig. G4 



92 



PRACTICAL APPLIED ELECTRICITY 



rent around the winding is counter clockwise, the end near- 
est you will be the N-pole and the other end, the S-pole. 

Another simple rule is to grasp the solenoid with the right 
hand with the fingers pointing around the coil in the direc- 
tion of the current, the thumb will then point toward the 
N-pole of the coil, as shown in Fig. 66. 





Fig. 65 



Fig. 66 



105. The Toroid.~If a solenoid is bent around until its 
two ends meet, or if a winding is placed on a ring, the 
arrangement thus produced will be a toroid. By winding 
the various turns closely and uniformly over the entire 
periphery of the ring, the lines of force produced inside 
the ring by a current in the winding will form closed curves i 
whose paths are entirely with- 
in the turns composing the 
winding; consequently, there 
are no externar magnet poles. 
Such a coil is shown in Fig. 67. 

106. Permeability. — The 
number of magnetic lines pro- 
duced inside a solenoid, with 
an air core, can be greatly in- 
creased by introducing a piece 
of iron, even though the cur- 
rent in the winding of the so- 
lenoid remains constant. This 
is due to the fact that the iron is a better conductor 
of magnetic lines than air. The relation between the num- 
ber of lines of force per unit area inside the solenoid after 
the iron has been introduced, designated by (^), to the 




Fig. 67 



ELECTROMAGNETISM 



93 



number of lines per unit of area for an air core, which is 
the field strength, designated by (H), is called the permea- 
bility, designated by (fi). 



fi = - 



(73), 



H 



The permeability of a given sample of iron is not constant 
because the value of (j8) does not increase at the same 
rate (E) increases. Curves showing the relation between 
the two quantities for wrought iron, cast iron, and cast steel 



CiiUUU - 




















"^ 












































































































































































cr 


























. 




















































vf*W 


;, V v: 


^ 




— 




— ' 




~" 


































nJ 


4i 


o;^&Vl^ 






































GT- 
















' 


- \ 5\^^ 


>\ 




___^ 




-— 






































^ 








Cr 


a^V?- 


r-* 


"^ 












































/ 






























































/ 






\^ 
























































/ 




/ 


























































C4 


f 


/ 






















L:. 


TA 








1 
























_ 






f 
















, t- 


U^ ^^J^ 




— 




"^ 


— 


"" 


■*" 






















1 




















1 — 


""^ 














1 




























1 






























































"in 








/* 


^ 




























































/ 






























































/ 






























































/ 






























































































































2000 


1 
















_ Valiie of 


:^ 


VmHi 


berts 






































u 


L- 


u 


u 


u 


L 




J 




u 


L. 


-J 


u 


: 



















10 20 30 40 50 60 70 80 90 100 HO 120 130 140 150 160 

Fig. 68 

are shown in Fig. 68. The permeability of these different 
kinds of iron can be determined for any value of (jS) or 
(fi) by dividing the value of (j8) for any point on the 
curve by the corresponding value of (fl). 

The sharp bend in the curve is called the "knee" of the 
curve. The iron is very nearly saturated at this point be- 
cause any further increase in (fi) produces a small increase 
in (j8) as compared to what a corresponding increase in 
{B.) would do below the knee of the curve. 

107. Magnetomotive Force. — The magnetomotive force 
(abbreviated m.m.f.) of a coil carrying a current is its total 
magnetizing power. When a current passes around a core 
several times, as shown in Fig. 69, the magnetizing power 



94 PRACTICAL APPLIED ELECTRICITY 

is proportional both to the strength of the current and the 
number of turns in the coil. The product of the current in 
the coil and the number of turns composing the coil is 
called the ampere-turns. The magnetizing power of the 
current is independent of the size of the wire, the area of 

the coils, or their shape, and re- 
/- mains the same whether the 



-"^jsf f \[Li\l:^Alj^^l:\t^\i^: §< turns are close together or far 
^ -'^ U V VU'V apart. It has been found by 

^ experiment that one ampere 

Fig. 69 turn sets up 1.2566 units of mag- 

netic pressure. Hence, if (n) 
represents the numbjer of turns in the coil and (I) represents 
the current in amperes through each turn, the magnetomotive 
force is 

m.m.f. = 1.2566 X n X I (74) 

The gilbert is the unit in which magnetomotive force is 
measured. One gilbert is equal to (1 -^ 1.2566) ampere-turn. 
Magnetomotive force or magnetic pressure corresponds to 
electromotive force and electrical pressure* in the electrical 
circuit. 

The m.m.f. acting in any magnetic circuit encounters a 
certain opposition to the production of a magnetic field, 
just as an electrical pressure encounters a certain opposi- 
tion in the electrical circuit to the production of a current. 
The opposition in the magnetic circuit is called the reluc- 
tance, represented by (B), oTlKe circuit and its value will 
depend upon the materials composing the circuit and the 
dimensions of the circuit. 

The total number of magnetic lines of force, called the 
magnetic flux, represented by (<l>), produced in any magnetic 
circuit will depend upon the m.m.f. acting on the circuit 
and the total reluctance of the circuit, just as the current 
in an electrical circuit depends upon the electrical pressure 
acting upon the resistance o*f the circuit. The unit of mag- 
netic flux is the maxwell, and it is equal to one line of force. 
The gauss is the unit of flux density, and it is equal to one 
line of force per unit of area. 

Magnetomotive force 

Number of lines = (75)^ 

Reluctance 




ELECTROMAGNETISM 95 



(76) 



108. Reluctance. — The reluctance of any magnetic cir- 
cuit depends upon the dimensions of the circuit and the 
kind of material composing the circuit. It varies directly 
as the length of the circuit and inversely as the area, all 
other conditions remaining constant. The reluctance of any 
given volume varies inversely as the permeability of the 
material filling the volume. 

Length in centimeters 

Reluctance = 

Permeability X cross-section in sq. cm. 

I 

B = (77) 

/JL A 

The unit in which reluctance is measured is called the 
oersted and it is equal to the reluctance of a cubic centi- 
meter of air. 

Reluctances can be added in the same way as resistances. 
If a magnetic circuit, such as that of the dynamo, is com- 
posed of a number of different kinds of materials, such as 
cast iron, wrought iron, air, etc., calculate the reluctance 
of each part by the above equation, and add these reluc- 
tances together to give the total reluctance of the entire 
magnetic circuit. The value of the permeability to use in 
the above equation will, of course, depend upon the number 
of magnetic lines the various parts of the circuit are to 
conduct per unit of area. The permeability of air is always 
unity. 

Example. — An iron ring has a rectangular cross-section of 
four square centimeters and a mean length of 20.5 centi- 
meters. A slot is cut in this ring .5 centimeters wide and 
,the ring is wound with 1000 turns of wire. What current 
must there be in the winding in order that there will be 
100 000 magnetic lines produced in the air gap? Take the 
permeability of the iron equal to 1000. 

Solution. — The reluctance of the iron portion of the cir- 
cuit can be determined by substituting in equation (77): 



itlhi 



96 PRACTICAL APPLIED ELECTRICITY 

20 1 



:r- 



oersted 



1000 X 4 200 
The reluctance of the air gap will be 

.5 
B = — =^ 1^ oersted 



1X4 



Total reluctance is 
1 



+ ■ 



.13 oersted 



8 200 ICO 
Substituting the values of (i?), (n), and (^) in equatio: 
(7.4) and (76) gives 

L2566 X 1000 X I 

100 000 = 

.13 
1.2566 X 1000 X I = 13 000 
1 256.6 I = 13 000 
1 = 10.34 



Ans. 10.34 amperes. 



Example. — The magnetic 
circuit, shown in Fig. 70, 
is rectangular in cross-sec- 
tion, the dimensions per- 
pendicular to the paper be- 
ing 3 centimeters. The 
permeability of the iron is 
1000. The armature (A) 
is .5 centimeter from eacli 
magnet core. There are 
500 turns in each coil and 
each turn is carrying a 
current of 10 amperes. 
What is the value of the 
total number of magnetic 
lines produced in the air 
gaps? 




Fig. 70 



Solution. — The total length of path in air is 
2 X .5 = 1 cm. 



ELECTROAIAGXETISM 97 

The area of the magnetic circuit is the same throughout 
and is equal to 

2 X 3 = 6 sq. cm. 

The reluctance of the air gaps is equal to 

I 1 1 



E 



fxA 1X6 6 

The length of the magnetic circuit in the iron is 

TT X d 

(2 X 5) + (2 X 10) + 4 ( ) = 33.1416 cm. 

4 

(Note — The circuit is taken as a curve about the corners.) 
The reluctance of the iron portion of the magnetic circuit 
will be 

33.1416 

E = = .005 523 6 oersted 

1000 X 6 

The total reluctance will be 
1 

h .0055236 = .17219 oersted 

6 

Substituting 'the values of (I), (n), and (R) in equation (76) 
gives 

1.2566 X 2 X 500 X 10 

^ = = 72 970. 

.17219 
Ans. 72 970. maxwells (approx.). 

PROBLEMS ON MAGNETISM 

(1) A magnetic circuit is 40 cm. in length, has a cross- 
section of 4 square centimeters, and is composed of a material 
whose permeability is 1500. What is the reluctance of the 
circuit? 

1 

Ans. oersted. 

150 



98 PRACTICAL APPLIED ELECTRICITY 

2. What is the m.m.f. in gilberts produced by a current 
of 7.5 amperes through a winding around a magnetic cir- 
cuit of 1200 turns? 

Ans. 11 309. + gilberts. 

(3) A magnetic circuit is composed of three parts con- 
nected in series, having reluctances of .032, .015, and .053 
oersted, respectively. What m.m.f. would be required to 
produce a flux of 10 000 maxwells? 

Ans. 1000 gilberts. 

(4) How many turns would be required in a coil to pro- 
duce the above m.m.f., if each turn is to carry a current of 
1 ampere? 

Ans. 795 turns (approx.). 

(5) It is desired to magnetize a piece of iron until there 
are 6000 lines per square centimeter. What cross-section is 
required to have a total of 100 000 lines? 

Ans. 16% sq. cm. 

(6) Two magnetic circuits are acting in parallel and they 
have reluctances of .05 and .04 oersted respectively. What 
is the total reluctance of the two combined? 

Ans. .0222 oersted. 

(Note: Add reluctances in parallel the same as resist- 
ances.) 

109. Electromagnet. — A simple electromagnet consists of 
a piece of iron about which is wound an electrical con- 
ductor through which a current of electricity may be passed. 
Commercial electromagnets assume numerous different forms 
depending upon the particular use to which they are to be 
placed. They are used in electric bells, telephones, relays, 
circuit breakers, generators, motors, lifting magnets, etc. 
The use of the electromagnet in handling magnetic materials 
has become quite common in recent years. A magnet manu- 
factured by the Electric Controller and Supply Company, 
Cleveland, Ohio, which is used for the above purpose, is shown 
in Fig. 71. 

110. Hysteresis. — If a piece of iron be magnetized, then 






ELECTROMAGNETISM 



99 



demagnetized and magnetized in the opposite direction and 
again demagnetized it will be found that the degree of 
magnetization will be different for the same value of (H) 
depending upon whether the field is increasing or decreasing 
in strength. The magnetization of the iron lags behind the 
magnetizing force and, as a result, the values of i^) for 




Fig. 71 



certain values of (H) will be greater when the magnetizing 
force is decreasing than they will be for the same values of 
(H) when the magnetizing force is increasing. Fig. 72 
shows the relation between (/3) and (H) when the iron 
is carried through what is termed a complete cycle; that is, 
it is magnetized to a maximum positive {(3), as at (a) in 
the figure; then demagnetized and magnetized to a maximum 
negative (13), as at (b) in the figure, which gives the upper 
curve (acb). The lower curve (b d a) is obtained in a 
similar way by demagnetizing the sample from a negative 
(/3) to zero and then magnetizing it to a maximum posi- 
tive (/3), returning the iron to its original magnetic condition, 
which completes the cycle. 

111. Hysteresis Loss. — When a piece of iron is carried 
through a magnetic cycle, as described in the previous sec- 



100 



PBACTICAL APPLIED ELECTRICITY 



I 




Fig. 72 



tion, all the energy spent in magnetizing it is not returned to 

the circuit when the iron is demagnetized, which results in 

a certain amount of electrical 
energy being expended to 
carry the iron through the 
cycle. This energy appears 
in the iron as heat. The en- 
ergy lost per cycle depends 
upon the kind of iron being 
tested, the volume of the 
sample, and the maximum 
value of (jS) raised to the 1.6 
power. 

Joules (energy per cycle) ==^ 
V X^i.6 XvX 10-T (78) ■ 

The constant (r)) takes into 
account the kind of iron being 
tested and (V) is the volume 
in cubic centimeters. If the 

iron is carried through (f ) cycles per second, the loss of power 

in watts is given by the equation 

Wh = <»? X f X V X /31.C X 10-7 watts (79) 

TABLE NO. VI 

VALUE OF HYSTERETIC CONSTANT (tj) FOR DIFFERENT 
MATERIALS 

Best annealed transformer sheet metal 001 

Thin sheet iron (good) 003 M\ 

Ordinary sheet iron 004 fl 

Soft annealed cast steel 008 fl 

Cast steel, 012 ■ 

Cast iron 016 ^1 

Example. — A piece of iron is magnetized to a maximum (p) 
of 9000 lines per sq. cm. It is carried through 60 complete 
cycles per second. What is the power lost in 10 cubic centi- 
meters if the hysteretic constant (yj) is .003? 

Solution. — In order to raise the value of (/3) to the 1.6 
power you must make use of logarithms. (A description of 
the use of logarithms is given in Chapter 20.) In the table 
of logarithms you will find opposite the number 900 the 
mantissa 95424. (The mantissa of the logarithm of 900 is 



ELECTROMAGNETISM 101 

the same as the mantissa of the logarithm of 9000.) 
Place the figure 3, the characteristic, before this num- 
ber, which is one less than the number of significant figures 
in 9000 and you have the log 9000 = 3.95424. Multiply 
this log by 1.6 and you have 6.326 784. Now 6.326 78 is the 
log of the result you want to obtain. Looking up the man- 
,tissa 32678 in the table, you find it corresponds to 2122. 
The result must contain seven fig.ures before the decimal 
point (because the characteristic is six), hence, the result 
is 2.122 X 106. Substituting this value in equation (79), 
together with the values of (f), (V), and (i?), gives 

Wh = .003 X 60 X 10 X 2.122 X 106 x 10-T 
= .18 X 2.122 = .38 

Ans. .38 watts. 

112. Law of Traction. — The formula for the pull of, or lift- 
ing power of, an electromagnet when it is in actual contact 
with the object to be lifted is 

/32A 

Pull in pounds = (80) 

72 134 000 
In the above equation (j8) is the number of lines per square 
inch and (A) is the area of contact in square inches. The 
value of (jS) required to produce a given pull, when the area 
of contact is known, can be calculated by the use of the 
equation 

Pull in pounds 

iS = 8494 (81) 

Area in square inches 
In the above equation ip) will be lines per square inch. 



CHAPTEE VII 

ELECTROMAGNETJC INDUCTION— FUNDAMENTAL 
THEORY OF THE DYNAMO 

113.' Electromagnetic Induction. — In 1831, Michael Faraday 
discovered that an electrical pressure was induced in a con- 
ductor that was moved in a magnetic field, when the direction 
of motion of the conductor was such that it cut across the 
lines of force of the field. If this conductor forms part of a 
closed electrical circuit, the electromotive force induced in it 
will produce a current. Currents that are produced in this 
way are called induction currents and the phenomenon is 
termed electromagnetic induction. In this great discovery 
lies the principle of the operation of many forms of commer- 
cial electrical apparatus, such as dynamos, induction coils, 
transformers, etc. 

114. Currents Induced in a Conductor by a Magnet. — If a 
conductor (AB), Fig. 73, that is connected in series with a 
galvanometer (G), located so that it is not influenced to any 
great extent by the permanent magnet, be moved in the 
field of the magnet, the moving system of the galvanometer 
will be deflected to the right or left of the zero position. 
This deflection is due to a current in the circuit which is 
caused by the induced e.m.f. in the conductor that was 
moved in the magnetic field. When the movement of the 
conductor in the field ceases, the galvanometer system will 
return to its zero position, which indicates there is no cur- 
rent and hence no induced e.m.f. in the circuit. Hence, the 
conductor must be actually cutting the magnet lines of force 
in order that there be an induced e.m.f. produced in the 
circuit. If the conductor was moved downward across the 
field in the above case ard the defiection of the galvanom- 
eter needle was to the right, it will be found, upon moving 
the conductor upward across the field or in the opposite direc- 

102 



ELECTEOMAGNETIC INDUCTION 



103 




Fig. 73 



tion to its motion in the first case, that the galvanometer 
needle will be deflected to the other side of its zero posi- 
tion. Since the direction in which the needle of the galva- 
nometer is deflected depends upon the direction of the current 
through its winding, it is apparent the current in the circuit 
in the second case is in 
the opposite direction to 
what it was in the first 
case; and since the current 
is due to the induced 
e.m.f. in the conductor 
(AB), it must also be in 
the opposite direction. If 
the motion of the conduc- 
tor in the magnetic field is 
continuous, up and down 
past the end of the mag- 
ret, there will be a current 
through the galvanometer 

first in one direction and then in the opposite direction, and 
the galvanometer needle will swing to the right and left of 
its zero position. The motion of the conductor, however, 
may be rapid enough so that the galvanometer needle has 
not sufficient time to take its proper position with respect 
to the current in the conductor and as a result it remains 
practically at zero, the vibration or defiection to the right 
or to the left being very small. 

The same results can be obtained by using the opposite 
pole of the magnet, except the deflection of the galvanometer 
needle due to a given direction of motion of the conductor 
will be just the reverse of what it was v/ith the other pole. 
This shows that there is some definite relation between the 
direction of motion of the conductor, the direction of the mag- 
netic field, and the direction in which the induced e.m.f. acts. 
If the wire were held stationary and the magnet moved, the 
same results would be obtained as though the wire were 
moved past the magnet. Hence, it is only necessary that 
there be a relative movement of the conductor and the 
field; either may remain stationary. An electromagnet may 
be used instead of the permanent magnet and the same 
results will be obtained under similar conditions. 



104 PRACTICAL APPLIED ELECTEICITY 

If the conductor were moved very slowly across the mag- 
netic field, the galvanometer needle would be deflected 
through a much smaller angle than it would be if the con- 
ductor were moved faster across the field. The deflection 
of the galvanometer needle depends upon the value of the 
current through its winding and since the deflection is 
smaller when the conductor is moved slowly than it is when 
the conductor is moved fast, it must follow that the induced 
e.m.f. for a slow movement of the conductor is less than it 
is for a fast movement, even though all the magnetic 
lines of force forming the magnetic field be cut by the con- 
ductor. The above results show that the value of the 
e.m.f. induced in a conductor, due to the relative motion of 
the conductor and a magnetic field, depend upon the rate 
at which the conductor is moving. If a second conductor 
be connected in series with the first, so that their induced 
e.m.f.'s act in the same direction, the resultant e.m.f. is 
increased. This is equivalent to increasing the effective 
length of the conductor in the magnetic field. 

The induced e.m.f, may be increased by placing a second 
magnet along the side of the first so that their like poles 
are pointing in the same direction. This second magnet in- 
creases the strength of the magnetic field and the conductor 
cuts more lines of force due to any movement. 

If the conductor be moved in a path parallel to the lines of 
force forming the magnetic field or along its own axis, there 
will be no deflection of the galvanometer needle, which indi- 
cates there is no current in the circuit and hence, no in- 
duced e.m.f. Then, in order that there be an induced e.m.f. 
set up in a conductor due to its movement with respect to a 
magnetic field, the path in which the conductor moves must 
make some angle with the direction of the magnetic lines of 
force. The value of the induced e.m.f. due to the movement 
of a conductor in a magnetic field will increase as the angle 
between the direction of the lines of force and the path 
in which the conductor moves increases, and it will be a 
maximum when the conductor moves in a path perpendicular 
to the direction of the magnetic field and perpendicular to 
itself. 

There will be an induced e.m.f. set up in the conductor 
even though the circuit of which the conductor forms a part 



4 



ELECTROMAGNETIC INDUCTION 105 

be open. This induced e.m.f. will exist between the ter- 
minals of the circuit where it is opened, just the same as an 
e.m.f. exists between the terminals of a battery that is on 
open circuit. 

The question naturally arises: Is the magnet weakened 
when it is used in producing induced currents in a conductor 
as previously described, and if not, what is the source of 
energy that causes the current to exist in the conductor? 
The magnet is in no way weakened when it is used as pre- 
viously described, and the induced current is produced by 
the expenditure of muscular energy just as an expenditure 
of chemical energy in a cell produces an electrical current 
in a closed circuit to which the cell is connected. When 
a conductor with a current in it is located in a magnetic 
field in a position other than parallel to the field, there is 
a force produced which tends to cause the conductor to move 
across the field. The direction of this force is just oppo- 
site to the one that must be applied to the conductor to 
cause it to move so there will be an induced e.m.f. set up 
• which will produce the current. In other words, the induced 
e.m.f. set up in a conductor will always be in such a direc- 
tion that the current produced by it will oppose the motion 
of the conductor. 

115. Currents Induced in a Coil 
by a Magnet. — If a coil of wire (C) 
be connected in series with a gal- 
vanometer (G), as shown in Fig. 
74, a deflection of the galvanom- 
eter needle can be produced by 
thrusting a magnet (M) in and out 
J of the coil. When the magnet is 
thrust into the coil, a deflection of 
^the needle will be produced, say, to ^^^- ^^ 

the right, and when the magnet is 

^withdrawn a deflection of the needle will be produced to the 
ileft. If the magnet be turned end for end, the deflections of 
the galvanometer needle will be just the reverse of what 
^they were for the previous arrangement. If the coil (C) be 
turned through an angle of 180 degrees — so that the side that 
svas originally toward the magnet will now be away from it^ 
and the magnet be placed in its original position, the deflec- 




106 PRACTICAL APPLIED ELECTEICITY 



II 



tions of the galvaDometer needle produced by a movement 
of the magnet in or out of the coil will correspond in direc- 
tion to those produced when the coil was in its original 
position and the magnet had been turned end for end. 

If the coil be moved on or off of the magnet, the same effect 
is produced as would be produced by moving the magnet in or 
out of the coiL The e.m.f. in this case, as in the previous 
one, will depend upon the rapidity of the movement of the 
magnetic field with respect to the coil, or vice versa. 

If the number of turns of wire composing the coil be 
increased or decreased, there will be a corresponding increase 
or decrease in the e.m.f. induced in the winding due to a 
given movement of the coil or magnet with respect to the 
other. The induced e.m.f. in the various turns all act in 
the same direction, they are all equal, and the resultant 
e.m.f. is equal to their sum. 

116. Magnitude of the Induced E.M.F. and Factors upon 
Which It Depends. — From the discussion in the two previous 
sections it is seen that the induced e.m.f. in a circuit de- 
pends upon the following factors: • jfli 

(a) The rate of movement of the conductor and the mag- 
netic field with respect to each other. The more rapid 
the movement the greater the e.m.f. induced, all other 
quantities remaining constant. 

(b) The strength of the magnetic field or the number of 
lines of force per square centimeter. The stronger 
the field the greater the e.m.f. induced, all other 
quantities remaining constant. 

(c) Upon the angle the path, in which the conductor moves, 
makes with the direction of the lines of force. The 
nearer this path is to being perpendicular to the mag- 
netic field and the position of the conductor, the greater 
the induced e.m.f. 

(d) The length of the wire that is actually in the magnetic 
field. The more wire there is in the magnetic field, the 
greater the induced e.m.f. 

The above facts can be condensed into the following 
simple statement: The magnitude of the induced e.m.f. 
in any circuit depends upon the rate at which the conductor. 



ELECTROMAGNETIC INDUCTION 107 

forming part of the circuit, cuts magnetic lines of force; that 
is, it depends upon the total number of lines of force cut per 
second by the conductor. When the conductor cuts one hun- 
dred million (100 000 000) lines in each second during its mo- 
tion, an electrical pressure of one volt is induced in the con- 
ductor. If the conductor cuts lines of force at the rate of 
two hundred million (200 000 000) in each second, the in- 
duced pressure is equal to two volts ; and if the conductor cuts 
eleven thousand million (11 000 000 000) lines in each second, 
there will be an induced e.m.f. of 110 volts. If the circuit of 
which this conductor forms a part be closed, there will be a 
current in the conductor, which has a strength equal to the 
induced pressure divided by the total resistance of the 
circuit. 

Example. — ^A conductor cuts across a magnetic field of 
110 000 000 lines of force 100 times per second. (The con- 
ductor always moves across the field in the same direction.) 
How many volts are induced in the wire? 

Solution. — A conductor cutting 11000 000 lines of force 
100 times per second would be equivalent to cutting 
(11000 000X100), or 1100 000 000 lines once per second. 
Cutting 1 100 000 000 lines of force per second will induce in 
a conductor (1 100 000 000 -^ 100 000 000), or 11 volts. 

Ans. 11 volts. 

117. Direction of Induced E.M.F. — From the previous dis- 
cussion it is seen that the direction of the induced e.m.f. 
depends upon the direction of the magnetic field and the 
direction in which the conductor is moved with respect to 
the field. 

If a piece of copper be bent into the form shown by 
(ECDF), Fig. 75, and a second piece (AB) be placed across 
the first and the combination placed in a magnetic field as 
shown by the vertical arrows in the figure, there will be a 
current around the metallic circuit thus formed when the con- 
ductor (AB) is moved to the right or to the left of its initial 
position. When the conductor (AB) is moved it cuts across 
some of the lines of force and there is an induced pressure set 
up in it which produces a current. The direction of this 
current will be reversed when the direction of motion of 
(AB), or the direction of the magnetic field, is reversed. 



108 



PRACTICAL APPLIED ELECTRICITY 



When the wire is moved in the direction indicated by the 
arrow (K) in the figure, the end (B) is positive and the other 
end (A) is negative, or the potential of (B) is higher than that 
of (A). This difference in pressure between (B) and (A) 
will cause a current in the circuit, from (B) through (C) 
and (D) to (A) and from (A) to (B). The wire (AB) is the 
part of the circuit in which the electrical pressure is gen- 
erated and the electricity passes from a lower to a higher 
potential through this part of the circuit, just as the elec- 
tricity passes from the negative to the positive pole of the 
battery through the battery itself. 

A simple way of determining the direction of induced 
e.m.f. in a circuit, when the direction of motion of the con- 



C 



— ^^ 
A 

Fig. 75 



a 



D 




Fig. 76 



ductor and the direction of the magnetic field are known, is 
as follows: Suppose a conductor (C), Fig. 76, is moved to 
the right, as indicated by the arrov/, in a magnetic field whose 
direction is downward, as shown by the small arrow heads 
at the lower part of the figure. The lines of force might be 
thought of as elastic bands that are pushed aside when the 
conductor is moved in the field, but finally break and join 
again on the left side of the conductor, leaving a line linked 
around the conductor, as shown by the small circle (c). The 
direction of this line of force about the conductor is clock- 
wise, or it corresponds to a line produced by a current toward 
the paper. Hence, the current in the conductor is from the 
observer toward the paper. It must be remembered that the 
current is from a point of relatively low potential to one of 
higher potential in this part of the circuit. 



ELECTEOMAGNETIC INDUCTION 



109 



118. Rules for Determining Direction of Induced E.M.F. — 
There are a number of different ways of remembering the 
relation between the direction of the electrical current, the 
direction of the conductor's motion, and the direction of the 
magnetic field. One of the best rules is what is known as 
Fleming's *'Right-Hand Rule," and it is as follows: Place 
the thumb and the first and second fingers of the right hand 
all at right angles to each other. Now turn the hand into 
such a position that the thumb points in the direction of 
motion of the conductor, and the first finger points in the 
direction of the lines of force, then the second, or middle, 
finger will point in the direction of the current that is set 
up in the conductor by the induced pressure. An illustra- 
tion of the "Right-Hand Rule" is shown in Fig. 77. 




Motion 
Induced emf 
Fig. 77 




Fig. 78 



119. Primary and Secondary Coiis.—If a coil of wire (Ci) 
be connected in series with a galvanometer (G), as shown in 
Fig. 78, and a second coil (C2) that has its winding con- 
nected to a battery (B) be moved into or out of the coil 
(Ci), there will be a deflection produced on the galvanometer, 
just as though a permanent magnet had been used instead of 
the coil (C2). The coil (Ci), in which the induced e.m.f. is 
produced, is called the secondary and the coil (C2), in which 
the inducing current exists, is called the primary. 

There are a number of different ways of producing an 
induced e.m.f. in the secondary coil besides moving one coil 
with respect to the other. Four of these methods are as fol- 
lows: (Both coils are stationary and one surrounds the 
other, or they are both wound around the same magnetic cir- 
cuit). 

(a) By making or breaking the primary circuit. — Imagine 



PRACTICAL APPLIED ELECTRICITY 



® 



B 



B 



3 



two conductors (AB) and (CD), Fig. 79, that are parallel to 
each other and very near together but are connected in 
two electrically independent circuits. The conductor (AB) 
is in series with the galvanometer (G) and constitutes the 
secondary circuit. The conductor (CD) is connected in se- 
ries with a battery (B) and a switch (S) that can be used 
in opening and closing the primary circuit. When the primary 
circuit is completed by closing the switch (S), there will be 
a current through the conductor (CD) from (C) to (D). 
This current will produce a magnetic field about its path 
and the field around the conductor (CD) will cut the con- 
ductor (AB) which will result in an 
induced e.m.f. being set up in the 
secondary circuit that will send a 
current through the circuit from (B) 
to (A). The direction of this in- 
duced e.m.f. can be determined by 
means of the **Right-Hand Rule." 
There will be an e.m.f. set up in the 
secondary for a period of time cor- 
responding to the time required to 
establish the current in the primary. 
As soon as the primary current ceases to change in value, 
there will be no movement of the magnetic field and the con- 
ductor (AB) with respect to each other. 

If now the primary circuit be broken, the magnetic field 
surrounding the conductor (CD) will collapse, and as a result 
the conductor (AB) will cut the field again, but in the oppo- 
site direction to what it did when the current in the circuit 
(CD) was being established. There will be a current pro 
duced in the secondary that is practically constant in dura- 
tion if the primary circuit is made and broken a suflicient 
number of times. The conductors forming the primary and 
secondary circuits are usually wound into coils, and they 
may be placed side by side or one outside the other. The 
e.m.f. induced in the secondary, due to a certain change of 
current in the primary, can be greatly increased by winding 
the two coils on an iron core. The magnetic field +hat 
passes through the two windings, due to the current in the 
primary is a great deal stronger when they are placed on 
the iron core than it is when an air core is used and, as a 



Fig. 



ELECTROMAGNETIC INDUCTION 



111 



CD=<>=0 



result, a greater number of lines of force will cut the second- 
ary winding when the primary circuit is completed or broken. 

The induction coil consists of two windings, a primary and 
a secondary, placed upon an iron core with some sort of 
a device connected in the primary circuit for interrupting 
the primary current. See Fig. 80. The relation between the 
primary and the secondary e.m.f. is practically the same as the 
relation between the number of turns of wire in the primary 
and in the secondary windings. 

(b) Varying the strength of current in the primary. — This 
in reality is practically the same as the previous method, 
except the circuit is not entirely broken. Any change in the 
value of the primary current will result in a change in the 
magnetic field surrounding 
the primary winding, and 
as this field expands or 
contracts it will cut the 
conductor composing the 
secondary and, as a result, 
there will be an induced 
e.m.f. set up in the sec- 
ondary winding. The di- 
rection of this induced 
e.m.f. will depend upon 
whether the field is ex- 
panding or contracting, 

which, in turn, depends upon the change of current in the 
primary — whether it be increasing or decreasing. The 
telephone induction coil is a good application of this 
means of producing an alternating current in the secondary 
due to a change in the value of the current in the primary. 
The connections of a telephone coil are shown in Fig. 81. 
(S) and (P) represent the secondary and the primary wind- 
ings of the induction coil, which are usually wound, one out- 
side of the other, on an iron core composed of a bundle of 
small iron wires. (R) is the receiver that is connected in 
series with the telephone line and the secondary winding. The 
transmitter (T) is connected in series with the battery (B) 
and the primary winding. The construction of the trans- 
mitter is such that when the air is set in vibration about the 
transmitter, due to any cause, there will be a change in the 




Condenser 



Fig. Si) 



112 



PEACTJCAL APPLIED ELECTEICITY 




Fig. 81 



value of the resistance it offers to the current in the circuit 
of which it is a part. The vibration of the air then causes 
a varying current through the primary winding of the induc- 
tion coil, which, in turn, produces an e.m.f. in the secondary 
winding and as a result of this e.m.f. there will be a current 
over the telephone line that produces an effect on the re- 
ceivers both at the sending and the receiving stations. It must 
be understood that the diagram, Fig. 81, does not show the 
complete circuit of the telephone. 

(c) Reversing the cur- 
rent in the primary. — If a 
switch were constructed 
so that its operation would 
reverse the current in the 
primary winding, there 
would be an e.m.f. induced 
in the secondary winding 
due to a change in the 
magnetic field surrounding 
the two windings. This 

method is applied in practice in what is called a transformer. 
The switch, however, is not used, as the current in the pri- 
mary winding is an alternating current — a current that is re- 
versing in direction at regular intervals. The operation of the 
transformer will be taken up under the subject ''Alternating 
Current." 

(d) Moving the iron core about which the windings are 
placed. — The magnetic field produced by a given value of 
current in the winding of a coil will depend upon the kind of 
material composing the magnetic circuit, whether it be a 
material of high or low permeability. If the iron core upon 
which the windings are placed be moved so as to increase the 
reluctance in the magnetic circuit, there will be a decrease 
in the lines of force, and, as a result, there will be an induced 
e.m.f. set up in the secondary winding. If the core be moved 
so as to decrease the reluctance of the circuit, there will be 
an increase in the number of magnetic lines, or an increase 
in field strength, and an induced e.m.f. will be produced 
in the secondary winding in the opposite direction to that 
produced when the field strength decreased. This principle 
is employed in what is called the inductor type of alternating- 



ELrECTKOMAGNETIC INDUCTION 



113 



current generator. An e.m.f. is produced by rotating iron 
poles between the primary and the secondary windings, which 
changes the number of the lines of force through the sec- 
ondary due to the current in the primary. 

120. Mutual Induction. — The reaction of two independent 
electrical circuits upon each other is called mutual Induction. 
These circuits of course must be so placed with respect to 
each other that the magnetic field due to the current in 
either of them will produce an effect in the other. A good 
practical example of mutual induction is that of a telephone 
wire that runs parallel to, say, an electric-light circuit. 
The magnetic field surrounding the electric-light circuit cuts 
the telephone conductor and sets up in it an induced e.m.f. 
This induced e.m.f. will 

Disturbing Wire 



Z)( 



Tel 



ephoneCirc 

' ^ — 2 3 



)(L 



uit 



Fig. 82 



produce a current in the 
telephone circuit which in- 
terferes with the satisfac- 
tory operation of the tele- 
phone line. Often the 
conversation on one tele- 
phone circuit can be heard 
on another circuit due to 

this same cause. The wires composing the circuits should have 
their positions interchanged, as shown in Fig. 82. The e.m.f.'s 
induced in the two wires composing the telephone circuit are 
opposite in direction with respect to the telephone circuit 
and they will all exactly neutralize each other when the two 
wires are properly changed in position. The changing of the 
position of two wires is called a transposition. 

121. Self-induction. — If the value of the current in a wire 
forming a coil be changed in any way, there will be a change 
in the strength of the magnetic field surrounding the wire.- 
This change in strength of the magnetic field will produce an 
e.m.f. in the conductor in which the current is changing just 
the same as though the field were changed in strength by a 
current in an independent electrical circuit. This cutting of 
the wire by the magnetic field produced by a current in 
the wire itself is called self-induction. When a coil carry- 
ing a current has its circuit broken, there will be a spark 
formed at the break due to the induced e.m.f. This induced 
e.m.f. will depend upon the form of the coil and the kind 



114 



PKACTICAL APPLIED ELECTKICITY 



M 



M 



r 



J 



of material associated with the coil. A straight conductor 
will have a small e.m.f. induced in it when the circuit is 
broken, as the magnetic field surrounding the conductor is 
not very strong. If the conductor be bent into a coil the 
induced e.m.f. will be greater than that for the straight 
conductor, as very nearly all the magnetic lines of force 
produced by each turn cut all the other turns composing the 
coil, and the total number of lines that cut the winding is 
greatly increased. This induced e.m.f. can be further in- 
creased by providing the coil with an iron core, which in- 
creases the field strength due to a given current in the 
winding. 

In electric-gas lighting it is desired 
r . to have a circuit of large self-induc- 

tion so that there will be a good 
spark formed when the circuit is 
broken at the gas jet. The heat of 
this spark is suflacient to ignite the 
gas. The self-induction of such a 
circuit is increased by connecting in 
series with the battery and other 
parts of the circuit a coil wound upon 
an iron core. This kind of a coil is 
often spoken of as a ''kick coil." 

If a lamp (L) be connected across 
the terminals of an electromagnet 
(M) , as shown in Fig. 83, the lamp will 
burn very bright just for an instant after the battery circuit is 
opened. This is due to the induced e.m.f. set up in the winding 
of the electromagnet when the field contracts and cuts the 
various turns. Tjie e.m.f., of course, is only momentary, as the 
field soon disappears when the circuit is broken by opening 
the switch (S). The voltage the lamp is constructed to 
operate on and the battery voltage should be practically the 
same in order to give the best results. 

122. Inductance. — The inductance of any circuit depends 
upon the form of the circuit and the kind of material sur- 
rounding the circuit. There is an increase in the value of the 
inductance of a coil with an increase in the number of turns 
and an increase in the permeability of the material com- 
posing the magnetic circuit. A coil is said to have unit 



hV 



Fig. 83 



ELECTEOMAGNETIC INDUCTION 115 

inductance when an induced e.m.f. of one volt will be produced 
due to a change in the current in the winding of one ampere 
in one second. That is, if the current changes, say, from 
two to three amperes in one second and there is an induced 
e.m.f. of one volt, the coil is said to have unit inductance. 
The unit of inductance is the henry, and inductance is usually 
represented by the symbol (L). 

The inductance of any coil can be calculated by the use 
of the following equation when the dimension of the coil 
and other quantities are known: 

4X7rXn2XAtXA 

L = (82) 

109 X I 

In the above equation (n) is the number of turns of wire 
on the coil, dm) is the permeability of the material composing 
the magnetic circuit, (A) is the area of the magnetic circuit 
in square centimeters, and (l) Is the length of the magnetic 
circuit in centimeters. 

123. Lentz's Law. — A careful consideration of the ways by 
which induced currents may be produced, whether it be due 
to self or mutual induction, will result in the following sim- 
ple f?ct. In all cases of electromagnetic induction, the cur- 
rent produced by the induced e.m.f. will always be in such a 
direction as to tend to stop the cause producing it. Thus, 
if a magnet be moved toward a coil, the current in the coil 
will be in such a direction that the side of the coil toward 
the magnet will be of the same polarity as the end of the 
magnet toward the coil. This results in the induced current 
tending to stop the motion of the magnet. When the magnet 
is moved away from the coil, the current in the coil will be 
in the opposite direction to what it was before and the 
side of the coil toward the magnet is of the opposite polarity 
to the end of the magnet toward the coil, and as a result 
they attract each other, which tends to prevent the magnet 
being moved. 

If a coil of wire (Ci), in which there is a current, be moved 
toward a second coil (C2), that is, connected in series with 
a galvanometer (G), Fig. 84, there will be an induced e.m.f. 
set up in the coil (C2), which will cause a current through 



116 



PRACTICAL APPLIED ELECTRICITY 




II 
I 

I 



Fig. 84 



the galvanometer and thus produce a deflection of its needle. 
The current produced in the second coil (C2) will be in such 
a direction as to make the sides of the two coils toward each 
other of the same polarity. These two poles repel each 
other and thus there is a force opposing the movement of the 
coils toward each other. When the coil (Ci) is moved away 
from (C2), the sides of the two coils 
adjacent to each other are of opposite 
polarity and they attract each other. 
This results in a force which tends to 
prevent the coils moving apart. It 
must be remembered that this force 
of attraction or repulsion between the 
two coils is present only when there ^ 
is a current in both coils. fl 

If the two coils be wound on an iron 
core and the value of the current in the primary winding be 
changed, there will be a current in the secondary in such a 
direction as to oppose any change in the value of the magnetic 
field produced by the current in the primary. 

124. General Rules for Direction of Induced Pressures. — 

(a) If a primary coil be moved into a secondary coil, the 

current in the secondary, due to the induced e.m.f., 
will be in the opposite direction to the primary current. 

(b) If a primary coil be moved out of a secondary coil, the 

current in the secondary, due to the induced e.m.f., will 
be in the same direction as the primary current. 

(c) Where the current is increasing in value in the primary 

coil, there will be a current in the secondary coil, due 
to the induced e.m.f. that is in the opposite direction 
to that in the primary coil. 

(d) When the current is decreasing in value in the primary 

coil, there will be a current in the secondary coil, 
due to the induced e.m.f. that is in the same direction 
as that in the primary coil. 

In the above cases the secondary is closed. If the secondary 
be open there will be an induced e.m.f. set up, which would 
produce a current in the direction indicated above, if the 
circuit was closed. Rules (c) and (d) apply when the 



ELECTROMAGNETIC INDUCTION 



117 



primary and the secondary are stationary and the current is 
changing in value in the primary. 

125. Eddy Currents. — If a disk of copper, or other conduct- 
ing material, be rotated below a suspended magnet, as shown 
in Fig. 85, currents will be produced in the disk, which cir- 
culate in paths similar to those shown by the dotted lines 
in the figure. These currents tend to oppose the motion pro- 
ducing them and, as a result, the magnet, if it is free to move. 





Fig. 85 



Fig. 86 



will be rotated in the same direction as the disk. If, how- 
ever, the magnet is held in position and the disk is rotated, a 
greater force must be applied to cause the disk to rotate than 
is required when the magnet is free to turn. 

Currents induced in masses of metal that are moved in a 
magnetic field, or are cut by a moving magnetic field, are 
called eddy currents. 

Faraday's dynamo, as shown in Fig. 86, consisted of a disk 
of copper (D) rotated between the poles of a permanent 
magnet (M), the electrical connection to the machine bemg 
made by means of brushes (Bi) and (B2) that rested upon the 
center and edge of the disk. 

126. Application of Eddy Currents. — A good example of 
the practical application of the fact that eddy currents are 
set up in a mass of metal revolved in a magnetic field, is found 
in almost all types of integrating wattmeters. A disk of copper 



118 



PRACTICAL APPLIED ELECTEICITY 



L the] 



is fastened on the same shaft as the rotating portion of 
meter is mounted upon, and this disk revolves between 
poles of several permanent magnets, as shown in Fig. 87. 
This combination of disk and magnets constitutes a small 
generator and serves as a load for the motor part of th^|| 



>f 

1 



meter. The torque required to drive the disk in the field 

of the magnets is propor- 
tional to the speed, and 
since the driving torque of 
the motor is proportional 
to the product of the im- 
pressed voltage and th< 
load current, or the watts, 
the speed of the moving 
part of the meter must be 
proportional to the watts. 
The integrating meter will 
be taken up more in detail 
in the chapter on "Electric- 
al Measuring Instruments." 
127. Eddy-Current Loss. 
— The energy expended in 
producing eddy currents 
is converted into heat and 
represents a loss. These 
losses are quite large in 
dynamos, motors, trans- 
formers, etc., and it is 
always best to reduce 
them to a minimum when 
it is possible. The best 
way of reducing them is to split the mass of metal up into 
sheets, the plane of these sheets being parallel to the direction 
of the lines of force. It is customary to build up all volumes 
of metal that are likely to have eddy currents produced in 
them from thin sheets, or laminations, as they are called. 
Induction-coil cores are made from short lengths of small wire 
instead of using a solid core. Armature cores and the cores of 
transformers are laminated so as to reduce the loss due to 
eddy currents. These laminae are usually between .014 and 
.025 inch in thickness for dynamos, and the space between 




Fig. 87 



ELECTEOMAGNETIC INDUCTION II9 

them that is taken up by the oxide that forms on their surface 
is about .002 inch. Losses due to eddy currents could be re- 
duced to an inappreciable value by decreasing the thickness of 
the laminae, but there is a practical limit on account of the 
decrease in effective iron area, caused by the waste of space 
taken up by the insulation between adjacent laminne. 

When the laminoo are perfectly insulated from each other, 
the following equation can be used in calculating the power 
in watts lost in iron due to eddy currents: 

Wo = k X V X f'^ X t2 x/32 (83) 

In the above equation (k) is a constant, depending upon 
the resistance of the iron per cubic centimeter, which is 
usually about 1.6 X lO-H; (V) is the volume of the iron in 
cubic centimeters; (t) is the thickness of one lamina in 
centimeters; (f) is the frequency of magnetic cycles per 
second; and ((3) is the maximum number of lines per square 
centimeter to which the iron is magnetized. 

Example. — Find the eddy-current loss in 1000 cubic centi- 
meters of iron, composed of laminations .04 cm. thick (in- 
cluding insulation), that is subject to a maximum (13) of 
10 000 lines per square centimeter and a frequency of 60 
cycles per second. 

Solution. — Taking the value of (k) = 1.6 X lO-n and sub- 
stituting in equation (83) gives 





1000 X 602 X (.04)2 X 10 0002 


"W — 




We — 


1.6 X 1011 




1000 X 3600 X .0016 X 100 000 000 


We^ 


1 


1.6 X 1011 




36 X .16 


We: 


= = 3.6 



1.6 

Ans. 3.6 watts. 

128. Non-inductive Circuit. — In the construction of certain 
coils it is desired to have them as nearly non-inductive as 
possible. This is accomplished by winding the coil with two 



120 



PRACTICAL APPLIED ELECTRICITY 



wires laid side by side, as shown in Fig. 88, their inner ends 
being joined electrically. When the winding is completedl 
the two outside ends form the terminals of the coil. The cur-l 
rent in such a coil is in one direction in one 
half of the turns and in the remaining half 
of the turns in the opposite direction. The 
result is that the magnetic effect of the 
current in one-half of the turns is equal 
and opposite to the other half and they 
exactly neutralize, and the inductance of the coil will be zero, 
or the coil will be non-inductive. 




Fig. 88 



CHAPTER VIII 

ELECTRICAL INSTRUMENTS AND EFFECTS OF A 
CURRENT 

129. Classification of Instruments. — No attempt will be made 
in this chapter to describe all of the various forms of instru- 
ments on the market at the present time, it being deemed 
best to confine the description in almost every case to 
those that are in most common use„ Electricity is not a 
material substance like water and cannot be measured in 
the same way since it has no dimensions such as length, 
breadth, or weight. An electrical current is studied and meas- 
ured by the effects it produces in an electrical circuit. The 
operation of all instruments depends upon some effect pro- 
duced by the current and this leads to the classification of 
instruments into four groups depending upon the particular 
effect employed in their operation. These effects are: 

(a) Electro-chemical effect. 

(b) Magnetic effect. 

(c) Heating effect. 

(d) Electrostatic effect. 

Note: The electrostatic effect is not an effect of a cur- 
rent primarily, but of electrical pressure. 

In addition to the above classification, instruments may 
be divided into the following groups: 

(a) Instruments suitable for direct-current measurements 
only. 

(b) Instruments suitable for alternating-current measure- 
ments only. 

(c) Instruments suitable for both direct- and alternating- 
current measurements. 

In the following discussion of instruments, they will be 
grouped according to the effect upon which their operation 

121 



122 



PRACTICAL APPLIED ELECTRICITY 



1 

direct 



depends, and their adaptability to the measurement of 

or alternating current will be pointed out at the same time. 



Ammeters, Galvanometers, and Voltmeters ■ 

130. Distinction between Ammeters, Galvanometers, and 
Voltmeters. — An ammeter is an instrument to be used in 
measuring the current in a circuit, and for that reason am- 
meters will always be connected in series with that part of 
the circuit in which it is desired to ascertain the value of 
the current. 

A galvanometer is an instrument used in detecting the 
presence of a current in a circuit, or for measuring the 
value of the current. It is really the same as an ammeter 
as far as construction and operation are concerned, but 
it is usually used in measuring very small currents as con 
pared to those measured by ammeters. 




Fig. 89 




Fig. 90 



In Fig. 89, the ammeter (A) is connected in series with 
the battery (B) and the two resistances (Ri) and (R2)» 
which are in parallel. The ammeter in this case reads 
the total current in the main circuit but it does not give 
the current in either of the resistances (Ri) or (Ro). It 
however, the instrument be connected, as shown in Fig. 90, 
it will indicate the current in the resistance (Ri). By chang- 
ing the instrument to the branch containing the resistance 
(R2), the current in this branch can likewise be determined. 

A voltmeter is an instrument for measuring the difference 
in electrical pressure between any two points to which 
its terminals may be connected. Thus, if it is desired to 
know the difference in electrical pressure between the ter- 



i 



ELECTKICAL INSTRUMENTS 123 

minals of any part of an electrical circuit, such as the differ- 
ence in pressure between the terminals of a lamp that is 
connected to some source of electrical energy, as shown in 
Fii". 91, the voltmeter is connected to the two terminals and 
its indication is a measure of the pressure over the lamp. The 
ammeter and the voltmeter both operate on the same prin- 
ciple, that is, their indications depend upon the current pass- 
ing through them. The voltmeter indication depends upon the 
pressure between its terminals, because the current through 
it varies with this difference in pressure, the resistance of 
the instrument remaining constant. 

The resistance of an ammeter should be very low for the 
following reason: An ammeter will always be connected 
directly in the circuit and there will be a difference in elec- 
trical pressure between its terminals when there is a cur- 
rent through it, which is at any instant numerically equal 
to the product of the current and the resistance of the instru- 
ment. This drop across the ammeter should be small in 
order that the power required to operate the instrument be 
low in value, it being equal to the product of the current 
through the ammeter and the difference in pressure between 
ammeter terminals. If the ammeter in Fig. 89 had a resist- 
ance whose value was something near the value of the com- 
bined resistance of the two coils (Rj) and (R2) in parallel, 
practically half the output of the battery would be consumed 
in the ammeter ard would represent a loss. If, on the other 
hand, the ammeter had a very low resistance, a very small 
part of the output of the battery would be consumed in it. 
The division of the current between the two branches of 
the circuit shown in Fig. 90 would be quite different after 
the ammeter was introduced in either branch to what it was 
before, if the ammeter had a large resistance. If the am- 
meter had a small resistance in comparison to that of either 
of the branches, the change in the division of the current 
between the two branches when the ammeter was introduced 
in either of them would be a great deal less than in the pre- 
vious case. 

The resistance of a voltmeter, on the other hand, should be 
as large as possible for the following reason: The loss 
of power in the voltmeter is equal to the product of the 
current through it and the difference in pressure between its 



124 



PEACTICAL APPLIED ELECTEICITY 



would result in 
by increasing the 



terminals, and since it is to indicate the difference in pres- 
sure, the only way this loss can be reduced is to increase the 
resistance of ihe instrument, which results in a smaller cur- 
rent. Suppose the voltmeter (V), shown in Fig. 91, had a re- 
sistance equal to that of the lamp (L), then the power 
required to operate the voltmeter would be equal to that 
required to operate the lamp, since there would be the 
same value of current in each, and the same difference in 
pressure between their terminals. This condition of affairs 
considerable loss and it could be reduced 
resistance of the voltmeter. An ammeter 
connected in the line lead- 
ing from the generator 
(G) to this lamp would in- 
dicate the combined cur- 
rent in the lamp and the 
voltmeter circuits. Hence, 
in order to get the current 
in the lamp circuit, the 
value of the current in the 
circuit should be subtracted from the value of 
current. If the resistance of the voltmeter be 
to that of the lamp, the current in its 




Fig. 91 



voltmeter 

the total 

large in comparison 

circuit will be small in comparison to the lamp current and 

the ammeter indication will be practically equal to that in the 

lamp circuit. The above discussion leads to the conclusion 

that an ammeter and a voltmeter need differ only in their 

resistance, the principle of operation being the same. 

131. Ammeter Shunts. — In the construction of ammeters it 
is usually customary to make them with two circuits between 
their terminals. One circuit has a very low resistance an^ ■ 
carries the greater portion of the current to be measuredBl 
while the other circuit has ^a comparatively large resistance 
and carries a small current. The current in the branch of 
larger resistance usually produces the deflection of the mov- 
ing system of the instrument. The other branch constitutes 
what is termed the ammeter shunt. The shunt and moving 
system are usually mounted in the same case, when the 
instrument is used in measuring currents ranging from a 
very low value to perhaps 600 amperes. For large currents 
they are usually constructed separately. 



ELECTRICAL INSTRUMENTS 



125 



If the resistance of the shunt is known, the current through 
it can be determined by measuring the difference in pres- 
sure between its terminals— by means of a suitable voltmeter, 
usually a millivoltmeter — and then dividing this difference 
in pressure in volts by 
the resistance gives the 
value of the current. 
Instruments used on 
switchboards as a rule 
have their shunts con- 
nected directly in the 
line, and small leads 
run from the terminals 
of this shunt to the 
moving system, which 
is mounted in a conven- 
ient place on the face 
of the board. An instru- 
ment and its shunt are 
chown in Fig. 92. The 
needle of this instru- 
ment comes to rest in 
the center of the scale 
when there is no cur- 
rent in its winding, and the direction of its deflection will 
depend upon the direction of the currents in the shunt. 




Fig. 92 



Instruments Whose Operation Depends Upon Electro-Chemical 
Effect of a Current 



132. Electrolysis. — If two conducting plates, such as plati- 
num, be immersed in acidulated water and a current of elec- 
tricity be passed through the solution from one plate to th^ 
other, the water will be decomposed into its two constituents, 
oxygen and hydrogen. Such a combination of plates and 
solution constitutes what is called an electrolytic cell, and 
the process of decomposing the liquid is called electrolysis. 
The solution through which the electricity is being conducted 
is called the electrolyte, and the two plates that are im- 
mersed in the solution are called electrodes. The plate by 



126 



PEACTICAL APPLIED ELECTRICITY 



which the electricity enters the solution is called the positive! 
electrode, or anode, and the plate by which the electricityj 
leaves the solution is called the negative electrode, or caxhodc 
The parts into which the electrolyte is decomposed are 
called ions. The ion liberated at the positive electrode is 
called the anion, and the one liberated at the negative pole 
is called the oath ion. The vessel, plates, and other appa-j 
ratus used in electrolysis constitute what is termed 
voltameter, when such apparatus is used to measure quantityj 
or current. When water is decomposed, hydrogen appears 
at the negative plate, or it is the cathion, and oxygen ap-' 
pears at the positive plate, or it is the anion. A simple 




Cathode 



Electrolyte 



Fig. 93 




Fig. 94 



electrolytic cell is shown in Fig. 93. The gas liberated at 
the two electrodes in this case passes off into the air. The 
voltameter, however, can be so constructed that the liberated 
gas will collect in an enclosed U-shaped tube, as shown in 
Fig. 94. The direction of the current in the circuit is inc'^.- 
cated in the figure by the arrow. The oxygen will collect 
in the tube (A) and force the water down, while the hydrogen 
will collect in (B) and force the water down. The weight of 
the oxygen gas liberated by a certain quantity of electricity 
will be about eight times that of the hydrogen gas, but the 
oxygen gas will occupy only half the space that the hydrogen 
gas occupies, since a given volume of hydrogen gas is 
approximately one-sixteenth as heavy as the same volume 



ELECTKICAL INSTKUMENTS 127 

of oxygen gas, and as a result the volume of gas collecting 
in the (B) tube will be practically twice the volume of gas 
in the (A) tube. 

133. Electrolysis of Copper Sulphate. — When a current ex- 
ists between two platinum plates immersed in a solution of 
copper sulphate CUSO4 made by dissolving copper sulphate 
crystals (bluestone) in water, the solution will be broken up 
by electrolysis into Cu (metallic copper) and SO4 (sulphion). 
The hydrogen gas liberated at the negative electrode takes 
the place of the Cu in the CUSO4 forming H2SO4 (sulphuric 
acid) and the Cu is deposited on the negative plate. Oxygen 
will be liberated at the positive electrode as in the previous 
case. All of the Cu contained in the solution will be de- 
posited on the negative plate if the current be allowed 
to pass through the electrolytic cell for a considerable time. 
When the Cu has all been removed from the solution it will 
be practically colorless. The reaction taking place in the cell 
can be represented as follows: 

CUSO4 = Cu + SO4 (84) 

Copper sulphate is decomposed into copper and sulphion. 
The Cu is deposited on the negative plate. 

SO4 + H2O = H2SO4 + O (85) 

The sulphion and water combine forming sulphuric acid and 
oxygen. The oxygen is liberated and passes off into the 
air and the sulphuric acid remains in the solution. 

In the above case the negative plate will increase in 
weight due to the deposit of copper upon it and this increase 
in weight is proportional to the quantity of electricity pass- 
ing between the two electrodes. 

If copper plates be substituted for the platinum plates, 
the sulphuric acid will attack the positive electrode and 
just as much copper will be thrown into solution as is 
deposited upon the negative electrode. This results in no 
change in the electrolyte, but there is a wasting away of the 
positive plate and an equal increase in weight of the negative 
plate as the electrolytic action continues. 

134. Electroplating. — The principles of electrolysis are ap 
plied in coating objects with a layer of metal and the proc. 
ess is called electroplating. The object to be plated always 



138 PEACTTCAL APPLIED ELECTRICITY 



P! 



forms the cathode of the electrolytic cell and the meta 
be deposited is held in solution and as the process con- 
tinues metal is supplied to the solution from a plate which 
forms the anode of the cell. This plate should, of course, 
be of the same material as the metal in the solution. All 
articles canrot be coated with certain metals without having 
first been coated with some other metal. As an example, 
articles composed of iron, tin, lead, and zinc cannot be silver- 
or gold-plated without first being copper-plated. 

135. Electrotyping. — An electrotype of a column of stand- 
ing type is made as follows : First, an impression in wax 
is made, then the surface of this impression is dusted over 
with powdered graphite to make the surface a conductor. 
This mold is then placed in a copper-plating bath, it forming 
the cathode, and receives a thin coating of metallic copper. 
When sufficient copper has been deposited, the mold is re- 
moved from the solution and the copper plate that was formed 
is separated from the mold and backed with type metal to a 
thickness of about % inch, and then mounted on a wooden 
block. It is necessary to back the copper plate with type 
metal because it is so thin that it would not stand the pres- 
sure to which it is subjected in the printing press. 

136. Polarity Indicator. — The polarity of a direct-current 
circuit can be determined as follows: Immerse the ends of 
two wires that are connected to the line wires, whose polarity 
it is desired to determine, into a vessel of water, taking 
care that the two ends do not come into contact with each 
other. Since approximately twice as much hydrogen gas 
(by volume) is liberated at the negative electrode as oxygen 
gas at the positive electrode, the polarity of the circuit being 
tested is easily determined. A simple polarity indicator can 
be constructed as follows: Place in a small glass tube a 
solution of iodide of potassium, to which a little starch has 
been added. Two short pieces of wire should be sealed 
into the ends of the tube. When a current is passed through 
this solution iodine is liberated at the positive terminal and 
the solution is turned blue around this terminal. 

137. Prevention of Electrolytic Action. — Electrolytic action 
is oftentimes the source of a great deal of trouble, especially 
in the deterioration of underground metals, such as gas, 
water, and sewer pipes, and the lead covering on telephone 



ELECTEICAL INSTRUMENTS 129 

and power cables. Electricity takes the path of least resist- 
ance in completing its circuit and, as a result, all conductors 
that are buried in the ground and not insulated from it, 
usually conduct some electricity, especially in localities where 
street car and power companies are operating with grounded 
circuits. There will be no decomposition of the pipe or 
lead sheath where the electricity flows onto them, but at the 
point whe^e it leaves, an electrolytic action will take place 
which results in the metal of the pipe or cable sheath being 
carried away. This, of course, means that the pipe or cable 
sheath will be greatly damaged or completely destroyed 
if the action is allowed to continue. To prevent this action, 
one end of a conductor is electrically connected to the pipe 
or cable sheath at the points where the electricity tends 
to leave them and the other end of the conductor is buried, 
or grounded. The electrolytic action still continues but it 
takes place at the end of the conductor in the ground and 
the damage is not serious. In some cases the pipes are 
wrapped with an insulating tape or coated with an insu- 
lating compound, which tends to prevent the electricity pass- 
ing on or off of them, thus reducing the electrolytic action. 
The connecting of a cable sheath or pipe to the ground is 
called bonding. Rail joints are bonded by connecting the 
ends of the rails electrically by means of a flexible copper 
conductor. 

138. Weight Voltameter. — Since the weight of a metal 
deposited or the weight of water decomposed by a given 
quantity of electricity is known, the electrolytic cell may be 
used as a quantity measuring instrument. Thus, if two cop- 
per plates be immersed in a solution of copper sulphate and 
a quantity of electricity passed between them, there will be a 
change in weight of the two plates. This change in weight 
of the plates can be determined and from it the quantity of 
electricity that passed between them can be calculated. An 
instrument of this kind can be used in measuring the current 
in a circuit, if it remains constant in value for a certain 
time. Thus, if a unit quantity of electricity pass between 
the two plates in one second, there will be a current of one 
ampere in the circuit. If ten units of quantity of electricity 
pass between the plates in ten seconds, there will still be a 
current of one ampere, or if ten units of quantity pass in 



130 PEACTICAL APPLIED ELECTKICITY 

two seconds, there will be a current of five amperes. Hence, 
in order to measure a current that is constant in value, with 
the voltameter, it is only necessary to note the time taken 
to produce a certain deposit. The weight of the total deposit 
divided by the time gives the weight of the deposit per sec- 
ond and this value divided by the weight each coulomb will 
deposit gives the current in amperes, since the current is 
the number of coulombs per second. Call the total gain in 
weight in grams (W), the time in seconds taken to produce 
the gain in weight (t), and the deposit produced by one 
coulomb (K). Then 

gain in weight in grams 

Amperes = 

gain in grams per coulomb X time in seconds 
or (86) 

W 

1 = (87) 

KXt 

Table No. VII gives the weight in grams that one coulomb 
or one ampere in one second will deposit (called the electro- 
chemical equivalent). 



TABLE NO. VII 
ELECTRO-CHEMICAL EQUIVALENTS IN GRAMS PER COULOMB 

K for silver 001118 gram 

K for copper 000329 gram 

K for zinc 000338 gram 

K for lead 001071 gram 

K for nickel 000304 gram 

A commercial form of voltameter is shown in Fig. 95. The 
two outside plates form the anode and they are connected 
electrically. There will be a metallic deposit on both sides 
of the middle plate which form the cathode, due to the pres- 
ence of the two outside plates. 

Example. — The increase in weight of a platinum plate in 
a silver voltmeter was 4.0248 grams when the circuit in 
which the voltameter was connected was closed for 15 min- 
utes. What current was there in the circuit? 



ELECTEICAL INSTRUMENTS 



131 



Solution. — Substituting in equation (87) gives 



4.0248 



I 



.001118 X 15 X 60 

Ans. 4 amperes. 

139. Adaptability of Voltameters. — The voltameter is really 
a quantity-measuring instrument and it will only measure the 
current when the rate of flow of electricity is constant and, as 
a result, it is not suitable for ordinary work. Determinations 




Fig. 95 



of the value of a current made by this instrument are very 
accurate, when properly made, and they are used as pri- 
mary standards in checking up the indications of other cur- 
rent-measuring instruments. 

The voltameter is not suitable for pressure measurements, 
since its resistance is not constant and, as a result, the cur- 
rent through it would not always be proportional to the pres- 
sure impressed upon the circuit of which it is a part. 
Voltameters can not be used in the measurement of an alter- 
nating current, since there would be a reversal of chemical 
action each time there was a change in the direction of the 



132 PRACTICAL APPLIED ELECTRICITY 

current. If the same quantity passed through the voltameter 
in one direction as passed through it in the other, there 
would be no metallic deposit on either electrode. 

Instruments Whose Operation Depends Upon Magnetic Effect 
of a Current 

140. Magnetic Effect of a Current. — The magnetic effect 
of a current was discussed in detail in the chapter on "Electro- 
magnetism" and it is only necessary to bear in mind the 
following facts: 

(a) There is a magnetic field about a conductor in which 
there is a current. 

(b) The direction of the magnetic field will depend upon 
the direction of the current in the conductor. 

(c) The strength or intensity of this field will vary with 
the value of the current in the conductor and the distance 
from the conductor. 

Instruments whose operation depends upon the magnetic 
effects of a current in a conductor may be classified as 
follows: 

A. Those in which permanent magnets are used, they 
being acted upon by the magnetic field produced by the cur- 
rent. Either the magnet or the conductor carrying the cur- 
rent may form the moving part. 

B. Those having soft iron parts which are moved due to 
the magnetic effect produced by the current in the con- 
ductor. 

C. Those in which no iron is used, but having two coils, 
one of which is movable. This coil is moved due to a mag- 
netic force that is exerted between them when there is 
a current in both coils. 

CLASS "A" 

141. Tangent Galvanometer or Ammeter. — If a magnetic 
needle be supported in the center of a coil of wire, as shown 
in Fig. 96, there will be a force tending to turn the needle 
from its position of rest in the earth's magnetic field when 
there is a current in the coil. This force will increase with 
an increase in current in the coil and, as a result, the needle 



J 




ELECTKICAL INSTKUMENTS 133 

will be deflected more and more as the current is increased 
until it occupies a position that is almost at right angles 
to its initial position. If a suitable scale be mounted, as 
shown in the figure, so that the ends of the needle, or a 
pointer that is fastened to the needle, will move over the 
scale, the deflection of the needle from the position of rest, 
due to a certain current, can be determined. 

The force which tends to return 
the needle to its initial position is 
due to the magnetic field of the 
earth. In using the ii?.strument, the 
coil carrying the current should be 
placed with its plane parallel to the 
plane of the needle and allowed to 
remain in this position while all the 
readings are being taken. Such an 
instrument is usually called a tan- 
gent galvanometer, or ammeter, ^..^ ^^^ 
because the value of the current Fig. 96 
through the coil is equal to some 

constant times the tangent of the angle through which the 
needle moves. The indications of an instrument of this kind 
are disturbed by the presence of magnetic fields other than 
that of the earth, or the current in the coil of the instrument 
itself, and as a result its operation is not very saisfactory 
except under ideal conditions. 

142. D'Arsonval Instrument. — In the D'Arsonval type of 
instrument, the permanent magnet is the stationary portion 
and the conductor carrying the current forms the movable 
part of the instrument. The instrument is named after a 
French scientist, who first put it into a useful form. The 
conductor carrying the current to be measured is bent into 
the form of a coil and may be either suspended or supported 
in the magnetic field of the permanent magnet. In the most 
sensitive forms of the instrument the coil is usually sus- 
pended by a conducting thread, such as phosphor-bronze or 
plated quartz fiber. The electricity is conducted to and 
from the coil by means of this support and another electrical 
connection at the bottom of the coil — which may consist of 
a second fiber of the same material as the upper one — or 
it may be a very fine wire coiled into a spiral, or a wire 



134 



PRACTICAL APPLIED ELECTRICITY 



3 



may dip into a cup of mercury. A D'Arsonval galvanometer 
is shown in Fig. 97. The deflections of the coil are measured 
by means of a telescope and suitable scale, together with 
a small mirror mounted on the moving system of the instru- 
ment. The image of the scale in the mirror may be read 
by means of the telescope, and as the mirror turns, the part 
of the scale visible through the telescope changes. 




Fig. 97 



An instrument of this kind can be constructed so that it is 
very sensitive and will respond to extremely small currents. 
It is not subject to outside disturbances to any great extent, 
such as stray magnetic fields, changes in the strength of the 
earth's magnetic field, etc. This instrument can be con- 
structed and adjusted so that it may be used in measuring 
either current or pressure. 

143. D'Arsonval Ammeters and Voltmeters. — The Weston 
ammeter and voltmeter are both good examples of D'Arson- 
val instruments. The moving coil in these instruments is 
mounted on pointed pivots that are extremely hard and 
rest in agate jewels, which results in a very small frictional 
resistance to the movement of the coil. A light pointer is 
attached to the coil and so arranged that it moves over a 
suitable scale properly graduated and lettered so that the 
indication of the instrument may be easily determined. A 
sectional view of the moving system of a Weston instrument 



ELECTRICAL INSTRUMENTS 135 

shown in Fig. 98, with a portion of the instrument cut 
away so as to show the construction. The permanent magnet 
is of the horseshoe type and has two pole pieces fastened 
to its ends by means of heavy screws. The inner surface 
of each of these pole pieces is cut so it forms an arc, the cen- 
ter of which is midway between the two inner surfaces of the 
magnet. A soft iron cylinder is mounted between the two 
pole pieces and serves to improve the magnetic circuit. This 



.^£^- 




Fig. 98 

cylinder is mounted on a piece of brass that is fastened to 
the two pole pieces, as shown in the figure, there being a 
small gap between the cylinder and the pole pieces. The 
coil is mounted so that it can turn about the cylinder. The 
force tending to return the coil to its zero position is sup- 
plied by two springs. These springs are so arranged that 
one winds up when the other unwinds, due to a movement 
of the coil. The electrical connection to the coil is made 
through these two springs. 

Practically the same moving system is employed in the 
construction of ammeters and voltmeters. In the ammeter, a 
low resistance, capable of carrying the current the instru- 
ment is supposed to measure, is connected between the bind- 
ing post on the instrument case; and the moving system is 



136 



PEACTICAL APPLIED ELECTRICITY 



connected in parallel with this resistance. Binding posts 
are provided of ample size to carry the current. A Weston 
portable ammeter is shown in Fig. 99. 

In the voltmeter, a re- 
sistance coil is con- 
nected in series with 
the moving system be- 
tween the terminals of 
the instrument. The 
value of the resistance 
to be placed in series 
with a moving system 
will depend upon the 
voltage a full scale 
reading of the instru- 
m e n t indicates. The 
range oi' any voltmeter 
may be increased by 
connecting additional resistances in series with it. Thus, if a 
resistance equal to that of the voltmeter itself be connected 
in series with the instrument, the voltmeter will indicate only 
half the pressure between the two points to which the 




Fig. 99 




Fig. 100 



I 



terminals of the combination are connected. Resistances, 
called multipliers, can be obtained from companies manufac- 
turing instruments to be used in increasing the range of a 
voltmeter. 

A contact key is usually mounted on each voltmeter case 
and so arranged that the circuit of the instrument may be 
opened or closed by manipulating the key. The instrument 






ELECTRICAL INSTRUMENTS 



137 



can be made so that a maximum scale deflection will cor- 
respond to one or more voltages. Thus an instrument may 
be constructed so that one connection will measure (0-3) 
volts and another will measure (0-150) volts. The resist- 
ance of these two circuits should be in the ratio of 3 to 
150. A Weston portable voltmeter is shown in Fig. 100. 

Instruments of the D'Arsonval type are suitable for gen- 
eral use because they are "dead-beat"; that is,, the coil or 
moving system goes immediately to its proper position with- 
out swinging back and forth, as in many of the other types. 

144. Adaptability of Instruments with Permanent Mag- 
nets. — Instruments whose operation depends upon a perma- 
nent magnet can be used only in direct-current measure- 
ments, because the direction of the deflections is dependent 
upon the direction of the current through them. 



CLASS "B" 

145. Plunger Type Ammeter or Voltmeter,' — The construc- 
tion of a plunger type ammeter or voltmeter is shown in 
Fig. 101. The coil (C) carries the current to be measured. 
There will be a magnetic force exerted on the rod of iron 
(R), when there is a current in the coil (C), which will tend 



Spring 
IcoiKC) 





Fig. 101 



Fig. 102 



to draw the rod into the coil, and thus cause the pointer 
(P) to move over the graduated scale (S). Gravity is the 
controlling force against which the magnetic force of the coil 
acts. The rod (R) and the coil (C) are both formed to the 
same curvature. The rod (R) is composed of very soft iron 
and it becomes a magnet due to the action of the current 



138 PKACTICAL APPLIED ELECTRICITY 

in the coil and, as a result, is drawn inside the coil. The 
distance the rod moves will depend upon the value of the 
controlling force and the ampere turns on the coil at any 
instant. In an ammeter, the coil (C) is wound with a few 
turns of large wire, while in a voltmeter it is wound with a 
large number of turns of small wire, the ampere-turns pro- 
ducing a given deflection in the two cases being the same. 
146. Magnetic Vane Ammeter or Voltmeter.— Instruments 
of this type operate on the principle that a piece of soft iron 
placed in a magnetic field and free to move will always 
move into such a position that it will conduct the maximum 
number of lines of force, or it will tend to move into the 
strongest part of the field and parallel to the field. Fig. 102 
shows a diagram of an instrument of this kind. The current 
to be measured is passed around the coil (C) producing a 
magnetic field through the center of the coil. A small piece 
of soft iron called the vane is mounted on a shaft that is 
supported in jewel bearings. This shaft is not in the exact 
center of the coil so that the distance the vane is from the 
inner edge of the coil will change as it moves about the 
shaft. The magnetic field inside the coil is strongest near 
the inner edge and, as a result, the vane will move so that 
the distance between it and the inner edge of the coil would 
be as small as possible, if it were not for a restoring force 
supplied by a coiled spring. The tendency of this vane 
to rotate increases with the current in the coil and, as a 
result, the pointer attached to the moving system moves 
over the scale with an increase in current. 

In some types of instruments there is both a stationary and 
a movable vane. They both become magnetized to the same 
polarity and, as a result, repel each other. This force causes 
the moving system to be deflected. Ammeters and volt- 
meters operating on this principle differ only in the size 
of wire and the number of turns in the coil (C). 

147. Thomson Inclined-Coil Instruments.— The construe- 
tion of a Thomson instrument is shown in Fig. 103. The coil 
(C) carrying the current is mounted at an angle to the 
shaft (S) supporting the pointer (P). A strip or bundle of 
strips of iron (I) are mounted on the shaft (S) and held by 
a spring, when there is no current in the coil, so that its posi- 
tion is nearly parallel to the plane of the coil. When a cur- 



ELECTKICAL INSTRUMENTS 



139 



rent is passed through the coil, the iron tends to take up a 
position with its longest dimension parallel to the magnetic 
field, which results in the shaft being rotated and the pointer 
moved over the scale. The degree of this movement will 
depend upon the value of the current in the coil. 

148. Adaptability of Instruments with Soft Iron Parts. — 
The instruments described in the previous three sections may- 
be used in measuring either direct or alternating currents and 
pressures. Their indications, however, are not reliable when 
used in direct-current work because they are influenced by 
outside fields and masses of iron. There is also a lag of the 
magnetic condition of the piece of soft iron behind the mag- 
netizing force, which results in their indications being lower 
than the true value as the current is increasing, and higher 




Fig. 103 

as the current is decreasing. With alternating currents, 
these objections are not present and the operation of the 
instruments will be found to be quite satisfactory, provided 
the outside field does not change at the same frequency as 
the current to be measured. 

CLASS "C" 



149. Electrodynamometer. — The electrodynamometer is, per- 
ihaps, the best example of an instrument whose operation 
depends upon the magnetic force exerted between two coils, 
both of which have a current in them. The two coils are 
usually placed at right angles to each other, as shown in 
Fig. 104, one of them being fastened rigidly to the frame of 
the instrument, while the other is supported or suspended and 
its position controlled by a spring. When a current exists in. 



140 



PRACTICAL APPLIED ELECTRICITY 



both coils, the movable one tends to turn into such a position 
that its magnetic field is parallel to the magnetic field pro- 
duced by the current in the stationary coil. This movable 
coil, however, is brought back to its zero position by turning 
the thumb screw, which is connected to the upper end of the 
spring, causing the spring to be twisted in the opposite direc- 
tion to that in which the coil tends to move. The tortion in 
the spring exactly balances the tendency for the coil to turn 
when the thumb screw has been turned through the proper 




Fig. 104 




angle. A pointer is fastened to this thumb screw and moves 
over a graduated scale. The effect produced by a given cur- 
rent through the two coils, which are connected in series, is 
then read as so many divisions on the scale. 

When the instrument is used as an ammeter, the two coils 
consist of a few turns of large wire, depending, of course, 
on the current capacity of the instriiment. Quite often the 
stationary coil is divided into two parts, only part of the 
turns being used in series with the movable coil for one 



ELECTEICAL INSTKUMENTS 141 

connection, while all the turns may be used if desired. The 
current required to produce a full scale deflection when 
all the turns in the stationary coil are in use will be less 
than the current required when only a portion of the turns 
are used. 

When the instrument is used as a voltmeter, the coils are 
composed of a large number of turns of small wire. Often an 
additional resistance is provided to be used in series with the 
instrument, which increases its range as a voltmeter. 

150. Adaptability. — The indications of instruments of the 
electrodynamometer. type are influenced by stray magnetic 
fields and, as a result, they are not altogether satisfactory 
for direct-current measurements. This error, however, can 
be reduced to practically zero, if the disturbing effect remains 
constant — which is not very often the case — by taking the 
average of two readings of the instrument with the current 
through it in opposite directions in the two cases. The above 
errors do not occur when instruments of the electrodyna- 
mometer type are used in alternating-current measurements 
— unless frequency of the distributing field is the same as the 
frequency of the current in the instrument — as the current 
is continuously reversing in direction. 

Instruments Whose Operation Depends Upon Heating Effect 
of a Current 

151. Heat Generated in a Conductor Carrying a Current. 
— When there is a current produced in a conductor, the 
energy expended in overcoming the conductor's resistance is 
manifested in the form of heat. Dr. Joule discovered that the 
heat developed in a conductor carrying a current was pro- 
portional to 

(a) The resistance of the conductor. 

(b) The square of the current in the conductor. 

(c) The time the current exists in the conductor. 

He also determined by experiment that 778 foot-pounds 
of work would raise the temperature of 1 pound of water 
1° Fahrenheit. This quantity, 778 foot-pounds, is called the 
meclianical equivalent of heat, or Joule's equivalent. A pound 
of water can be heated electrically by immersing a conductor 
carrying a current in the water until its temperature is raised 



142 PEACTICAL APPLIED ELECTRICITY 

1° Fahrenheit, or the same work will be done electri^^ally 
as was previously done mechanically. Joule found by experi- 
ment that 1 ampere under a pressure of 1 volt in a circuit for 
1 second, or 1 watt expended, would do the same work as 
0.7373 foot-pound expended in 1 second, or 

1 watt = 0.7373 foot-pound per second (88) 

The British thermal heat unit (abbreviated b.t.u.) is de- 
fined as the amount of heat required to raise one pound ol 
water 1° Fahrenheit at its maximum density (39.1° Fahren- 
heit). Since 778 foot-pounds of work is equivalent to 1 
b.t.u. and 1 watt is equal to .7373 foot-pound per second, then 
1 watt acting for 1 second will develop .000 947 7 b.t.u., or 

b.t.u. = .000 947 7 X (E X I) X t (89) 

Substituting for (E) its value, (I X R), gives 

b.t.u. = .000 947 7 X I2R X t 

Example. — What current must be passed through a resist- 
ance of 100 ohms immersed in 10 pounds of water in order 
that its temperature be raised 80° Fahrenheit in 5 minutes? 

Note: (All losses due to radiation are to be neglected and 
all the energy is supposed to be converted into heat). 

Solution. — There will be 80 b.t.u. required for each pound 
of water or a total of 80x10 = 800 b.t.u. Substituting in 
equation (89) gives 

800 = .000 947 7 X 12 X 100 X (5 X 60) 

800 8 

12 = = 

.000 947 7 X 100 X 300 .284 31 

12 = 28.13 
I =5.3 + 

Ans. 5.3-}- amperes. 

152. Commercial Applications of the Heating Effect of a 
Current. — Numerous practical applications are made of the 
heating effect of a current in a conductor, such as electric 
irons, cooking stoves, heaters, lamps, electric welding, fuses, 
etc. Since the heat generated in any part of a circuit is 
proportional to the resistance of the part considered, it fol- 
lows that it is always desirable to have all conductors used 



ELECTEICAL INSTRUMENTS 143 

in transmitting electrical energy of as low resistance as pos- 
sible in order that the loss in transmission be a minimum. 
On the other hand, when it is desired to generate heat, resist- 
ance is introduced and the value of the heat generated can be 
determined by the use of equation (89). If none of the 
heat generated in a circuit were carried away, the temperature 
of the conductor would continue to rise as long as there 
was a current in it. As a matter of fact, however, the tem- 
perature of a conductor will rise until the heat radiated is 
exactly equal to the heat generated in a given time. This 
rise in temperature may be excessive and for that reason 
the Fire Underwriters limit the value of the current a con- 
ductor should carry. Table G, in Chapter 20, gives the cur- 
rent-carrying capacity of different sized wires. These 
values may be exceeded without any injurious effect, but 
it is always essential to keep within the values allowed by 
the Underwriters as the likelihood of a fire due to an over- 
heated or burnt-out circuit is less than it would be if the 
circuit were overloaded. The size of the conductor used in 
electrical heating utensils is just sufficient to carry the 
current without being injured. 

In electric welding a large current is passed between the 
surfaces that it is desired to weld together and the heat 
generated is sufficient to raise the materials to a welding 
temperature. The heat generated in an incandescent lamp 
raises the temperature of the filament to such a value that 
it will emit light. 

The electric fuse is a device whose function is to auto- 
matically open a circuit in which there is an excessive cur- 
rent. The fuse usually consists of a material that will melt 
at a much lower temperature than the material compos- 
ing the remainder of the circuit. It is nothing more than 
a weak spot in the circuit which is capable of conducting 
the current the circuit is supposed to carry, but will melt 
when the current through it becomes excessive. The val- 
ues of the current required to fuse different sizes of wires 
is given in Table H, in Chapter 20. 

153. Hot-Wire Instruments. — The heat generated in a con- 
ductor due to a current in it will cause the conductor to 
expand. The amount of this expansion will depend upon 
the rise in temperature, which, in turn, depends upon the cur- 



144 



PRACTICAL APPLIED ELECTEICITY 




rent in the conductor. The principle on which h 
wire instruments operate is shown diagrammaticallj 
in Fig. 105. A wire (A B) of comparatively high re- 
sistance, low temperature coefficient, and non-oxidizable 
metal, has one end attached to the plate (C), then passed 
around a pulley (P) that is secured to a shaft (S), and its free 
end is brought back and mechanically, though not electrically, 
attached to the plate (C). The spring (F) keeps the wire 
under tension, it being attached to the plate (C), which is 
so guided that it can move in a direction at right angles to 
the shaft (S). An arm (G) is also 
attached to the shaft (S), it being coun- 
terweighted at the upper end and bifur- 
cated at the lower end. A fine silk thread 
(T) has one end attached to one of the 
arms of (G), then passed around a small 
pulley (H), which is mounted on a shaft 
that carries a pointer (I), and finally has 
its other end attached to the second arm 
of (G). The material composing the arms 
of (G) is springy and serves to keep 
the silk fiber in tension. The current 
to be measured passes through the wire 
(A), entering and leaving through two 
twisted conductors, as shown in the 
figure. When a current is passed through 
(A) it is heated and expands, which 
results in the tension in (A) being 
less than that in (B) — they were originally the same — and 
equilibrium can be restored only by the pulley (P) rotating 
in a clockwise direction. This rotation of the pulley (P) 
causes the lower end of the arm (G) to move toward the left, 
and the silk thread that passes around the pulley (H) causes 
it to rotate in a clockwise direction and, as a result, the«| 
pointer (I) is deflected to the right, it being rigidly attached 
to the pulley (H). The operation of this instrument is 
quite satisfactory, as any change in the temperature of the 
room in which it is used does not affect the correctness of 
its indications, since both parts of the wire (A B) are af- 
fected to the same extent, which results in no movement of i 
the pointer. In a great many instruments no adjustment or j 




Fig. 105 



ELECTKICAL INSTRUMENTS 145 

compensation is provided for errors due to changes in room 
temperatures. In the case of ammeters, a low-resistance in- 
strument is usually used in parallel with a suitable shunt, 
while* in the case of voltmeters a high resistance instrument 
is employed. 

154. Adaptability of Hot-Wire Instruments. — Hot-wire in- 
struments may be used equally well in both alternating and 
direct-current measurements, the heating effect of the cur- 
rent being independent of its direction. 



Instruments Whose Operation Depends Upon the Electro- 
static Effect 

155. Condenser. — Two conductors separated by an insu- 
lator, which is called the dielectric, constitute what is called 
a condenser. When the terminals of a condenser are con- 
nected to some source of electrical energy, such as a bat- 
tery or a dynamo, there will be a certain quantity of elec- 
tricity stored in the condenser. The value of the quantity 



< 



_ — ^ ^^v 

Plates Dielectric 

Fig. 106 Fig. 107 

Stored will depend upon the capacity of the condenser and 
the electrical pressure to which its terminals are subjected. 
A condenser is said to have a capacity equal to unity, or 1 
farad, when a unit of quantity, 1 coulomb, will produce 
a difference in pressure between its terminals of 1 volt. 
A simple form of condenser is shown in Fig. 106, which con- 
sists of two metallic plates separated by a sheet of paraf- 
fine paper. The capacity of such a condenser will depend 
upon the area of the plates used in the construction, the 
number of plates, the distance between the plates, or the 
thickness of the dielectric, and the kind of material com- 
posing the dielectric. The value of an insulating material 
as a dielectric is called its specific inductive capacity. The 



146 PEACTICAL APPLIED ELECTRICITY 



BrablyP 



inductivQ capacity of different materials varies considerably 
but it is less through air than it is through any solids or 
liquids. As a result, a condenser that has air for a dielec- 
tric will have less capacity than one that has a solid or 
liquid dielectric, the dimensions of the condensers being 
the same. The specific inductive capacity of a material is 
measured in terms of air as a standard, and it is the ratio 
of the capacities of two condensers, one of which has the 
material, whose dielectric constant is to be determined, as 
a dielectric, and the other has air as a dielectric. The di- 
mensions of the two condensers must be the same. Table 
No. VIII gives the value of the specific inductive capacity of 
some of the materials that are commonly used in the con- 
struction of condensers. 



TABLE NO. VIII 
SPECIFIC INDUCTIVE CAPACITIES 





Specific 




Specific 


Material 


Inductive 


Material 


Inductive 




Capacity 




Capacity 


Air 


1.00 


Porcelain 


4.38 


Alcohol (Amyl) 


15.50 


Resin 


2.52 


Glass (Plate) 


3.(X) to 7.00 


Rubber 


2.80 


Gutta-percha 


2.50 


Shellac 


3.35 


Mica 


G.70 


Sulphur 


2.50 to 3.80 


Paraffiue 


1.90 to 2.40 


Turpentine 


2.20 


Petroleum 


2.10 


Vaseline 


2.17 



Condensers used in practice usually consist of more than 
two plates, as shown in Fig. 106. An arrangement similar 
to that shown in Fig. 107 is usually used where alternate 
plates are connected together and form one terminal, while 
the remaining plates are connected together and form the 
other terminal. The capacity in farads of such a combina- 
tion of plates as that shown in Fig. 107 can be calculated 
by the use of the equation 

K(n — l)a 

C=- (90) 

4.452 X 1012 X d 

where (n) represents the total number of plates used in the 
construction of the condenser, (K) represents the constant of 
the dielectric, (a) represents the area of each plate in square 
inches, and (d) is the distance between the plates in inches. 



ELECTRICAL INSTRUMENTS 



147 



Example. — Determine the capacity of a condenser of 100 
sheets of tinfoil 10 X 10 inches, that are separated by plate 
glass whose dielectric constant is 5. and 0.1 of an inch in 
thickness. 

Solution. — Substituting the values of the above quantities 
in equation (90) gives 

5 X (100-1) X 10 X 10 

C (in farads) = = .000 000 111 2 

4.452 X 1012 X 0.1 

Ans. .000 000 111 2 farads. 

The farad is a unit of capacity entir.ely too large for prac- 
tical use so a smaller unit, or the microfarad, is used. The 
microfarad is equal to one-millionth of a farad. 



c. c, c. 



Fig. 108 




156. Connection of Condensers in Series and Parallel.— 
Condensers may be connected in series or in parallel, or in any 
combination of series and parallel, just as resistances. The 
capacity of a combination of condensers may be calculated 
by the use of one of the following equations. When a num- 
ber of condensers are connected in series, as shown in Fig. 
108, the total capacity (C) of the combination will be given by 
the equation 



1111 

— = — + — + — + etc. 
C Ci Co Co 



(91) 



This results in the combined capacity of several condensers 
in series being less than the capacity of any one of them. 



148 PEACTICAL APPLIED ELECTKICITY 

If there are only two connected in series, the combined cj 
pacity will be equal to 

C1C2 
C = (92) 

C1 + C2 

It is seen that the above equation is similar to the one used 
in calculating the combined resistance of a number of re- 
sistances in parallel. 

When a number of condensers are connected in parallel, 
as shown in Fig. 109, the total capacity (C) of the combina- 
tion will be equal to 

C = Ci + C2 + C3 + etc. (93) 

The combined capacity of several condensers in parallel 
will be greater than the capacity of any single condenser. 
This equation is similar to the one used in calculating the 
combined resistance of several resistances in series. 

157. Relation of Impressed Voltage, Quantity, and Con- 
denser Capacity. — If a condenser is said to have unit capacity 
when 1 coulomb of electricity will produce a difference 
in electrical pressure between its terminals of 1 volt, then 
1-volt pressure applied at the terminals will produce a charge 
of unit quantity, 1 coulomb, if the capacity is 1 farad. If 
1-volt pressure produces a charge of 2 coulombs, the capacity 
is 2 farads ; or if 1-volt pressure produces a charge of one-half 
coulomb, the capacity is one-half farad. If the capacity of a 
condenser is constant, the quantity of charge is directly pro- 
portional to the impressed voltage; or if the impressed voltage 
is constant, the quantity of charge is directly proportional 
to the capacity. These two statements may be combined in 
an equation which states that the quantity varies directly as 
the capacity and the impressed voltage, or 

Q = CE (94) 

The above equation gives the value of the quantity in cou- 
lombs stored in a condenser whose capacity is (C) farads, 
when it is subjected to a pressure of (E) volts. Since the 
microfarad is the more common unit of capacity, it being 
equal to the one-millionth part of a farad, the above equa- 



ELECTRICAL INSTRUMENTS 149 

tion may be rewritten with the value of (C) in microfarads, 
which gives 

Q = E (95) 

106 

It is often desirable to measure the quantity in a unit smaller 
than the coulomb, in which case the microcoulomb is used, 
it being the one-millionth part of a coulomb. 



PROBLEMS OF CONDENSERS 

(1) Calculate the capacity of a telephone condenser com- 
posed of 1001 sheets 'of tinfoil 3X6 inches, separated by 
paraffine paper .007 of an inch thick, and having a dielectric 
constant of 2. 

Ans. 1.15 + microfarads. 

(2) Two condensers of 4 and 6 microfarads, respectively, 
are connected in series. Calculate their combined capacity. 

Ans. 2.4 microfarads. 

(3) What quantity of electricity will be stored in the two 
condensers in problem 2 when they are connected in series 
and there is an electrical pressure of 100 volts applied to 
the terminals? 

Ans. .000 24 coulomb, or 240 microcoulombs, 

(4) Two condensers of 4 and 6 microfarads, respectively, 
are connected in parallel. Calculate their combined capacity. 

Ans. 10 microfarads. 

(5) What quantity in microcoulombs will be stored in 
each condenser when there is a pressure of 100 volts ap- 
plied to the terminals of the combination in problem 4? 

Ans. 400 microcoulombs in 4 m.f. condenser; 600 micro- 
coulombs in 6 m.f. condenser. 

(6) What pressure is required to charge a 2-microfarad 
condenser with a charge of .0002 coulomb? 

Ans. 100 volts. 

(7) What is the capacity of a condenser when 100 volts 
will give it a charge of .000 1 5 coulomb ? 

Ans. 1.5 microfarads. 



150 PEACTICAL APPLIED ELECTEICITY 



I 



158. Electrostatic Voltmeter. — The electrostatic voltmeter 
is really nothing more than a condenser constructed so that 
one plate may move with respect to the other. When a 
condenser is charged, there is a force tending to draw the 
plates together, due to the charges of opposite kind on rhe 
two sets of plates, and it is this force that produces the 
deflection in the case of an electrostatic voltmeter. In some 
instruments only two plates are used, while in others a 
number of plates are used to form each terminal, the con- 
struction being such that the movable plates move between 
the stationary plates. 




Fig. no Fig. Ill 

An electrostatic voltmeter suitable for measuring extremely 
high voltages is shown diagrammatically in Fig. 110. The 
principal parts are the moving elements (Mi) and (M2), the 
curved plates (Bj) and (B2), the condensers (Ci) and (C2), 
scale (S), pointer (P), and terminals (T^) and (T2). The 
plate (Bi) is connected to the inner plate of (Ci) and (B2) 
to the inner plate of (C2). The plates (Bi) and (B2) are 
so arranged with respect to (Mi) and (M2), which are elec- 
trically connected, that an angular deflection of the pointer 
over the scale in the positive direction (above zero) short- 
ens the gap between the moving elements and the fixed 
plates. The charges on the plates (B^) and (B2) are of op- 
posite sign and they induce opposite charges on (M^) and 



ELECTRICAL INSTRUMENTS 151 

(M2), which results in a force that tends to cause the mov- 
ing system to turn about its own axis, or into such a posi- 
tion that (Ml) and (M2) are nearer (Bi) and (B2). The 
movement, however, is restrained by means of a spiral spring 
and the deflection is indicated by the pointer (P) on the 
scale (S). The form of the plates (Bi) and (B2) is such 
that the deflection increases almost directly as the impressed 
voltage. The condensers (Ci) and (C2) are so constructed 
that either or both of them may be short-circuited, which 
results in the pressure required to produce a full scale de- 
flection being less than when they are in circuit, thus giving 
a wide range to the instrum^ent. The moving system is im- 
mersed in a tank of oil, which affords a good insulation and 
buoys up the moving system, practically removing all the 
weight from the bearings. Instruments of this type are con- 
structed to measure voltages up' to 200 000 volts. An in- 
terior view of the various parts is shown in Fig. 111. There 
are many other forms of electrostatic voltmeters on the mar- 
ket, but their operation is based on the same fundamental 
principle as the one just describedo 

159. Adaptability of Electrostatic Instruments. — Electro- 
static instruments may be used in measuring either direct 
or alternating pressures. Their operation is more satisfac 
tory, however, on an alternating-current circuit, because a.c. 
pressures are, as a rule, larger in value than d.c. pressures. 
Their operation is not at all satisfactory when the pres- 
sure to be measured is below 50 volts, and for this reason 
they cannot be used in combination with a shunt in the 
measurement of currents, as the resistance of the shunt 
would have to be large in order that there be the proper 
difference in pressure between its terminals to operate the 
meter. A high-resistance shunt of this kind in any circuit 
would mean a large loss in comparison to that which would 
occur if a low-resistance shunt could be used. 

Measurement of Electric Power and Construction of Watt- 
meters 

160. Measurement of Power. — The power that is used in 
any part of a direct-current circuit may be determined by 
measuring the current with an ammeter, and the difference 
in pressure, by means of a voltmeter, between the terminals 



152 



PEACTICAL APPLIED ELECTitlOITY 



of the portion of the circuit in which it is desired to ascer- 
tain the power. The power can then be calculated by multi- 
plying the ammeter reading, in amperes, by the voltmeter 
reading, in volts, or 

' W = IE (96) 

Thus the power taken by the ten incandescent lamps shown 
in Fig. 112 can be determined by connecting the ammeter 
(A) and the voltmeter (V), as shown in the figure, and noting 
their indication when the lamps are turned on. (All of the 
lamps are not shown). The product of these instrument 
readings, in volts and amperes, will give the power in watts. 
When the connections are made, as shown in Fig. 112, the 




Ten Lamps 



Fig. 112 



ammeter indicates the current through the lamps and volt- 
meter combined. This results in a small error in the cal- 
culation of the power taken by the lamps, provided the 
current through the voltmeter is small in comparison to 
the current through the lamps. For very accurate measure- 
ments, the current through the voltmeter should always be 
subtracted from the total current indicated by the ammeter. 
If the voltmeter be connected across the line before the 
ammeter — that is, the ammeter would be between the volt- 
meter connection and the lamps — the indication of the volt- 
jneter would not be the true value of the pressure between 
the terminals of the lamps, as there would be a certain drop 
in pressure across the ammeter. The drop across the am- 
meter should be subtracted from the voltmeter indication 
in order to obtain the true pressure across the lamps. With 



ELECTRICAL INSTRUMENTS 



153 



a low-resistance ammeter, this drop is very small and may 
be neglected except when very accurate results are desired. 

161. Principle of tlie Wattmeter. — Wattmeters are so con- 
structed that their indication is proportional to the product 
of a current and an electrical pressure; hence, they indicate 
the power direct. Instruments of this kind are called watt- 
meters, because they measure watts. 

The principle of the wattmeter can be illustrated by ref- 
erence to Fig. 113, which shows an electrodynamometer 
with one coil connected in series with the line and the other 
coil connected across the line. The coil connected in series 
with the line is wound as though it were to be used as an 
ammeter, and consists of a few turns of large wire, while 
the coil connected across the line is wound as though it were 
to be used as a voltmeter and consists of many turns of 
fine wire. These two coils will be called the pressure coil and 
the series coil, respectively. 

The actual operation of such a meter can best be explained 
by taking a practical example sim- 
ilar to that shown in Pig. 113. The 
current through the lamps passes 
through the current coil of the 
meter and produces a certain mag- 
netic effect, which changes in value 
as the current changes in value. 
There will also be a magnetic effect 
produced by the current that exists 
in the pressure coil. The current in 
the pressure coil, or circuit, will 
vary with the difference in pressure 
between the terminals of the circuit, 
since the resistance of the circuit 
remains constant and, as a result, 
the magnetic field about the pres- 
sure coil will vary With the voltage of the line. The deflection 
of the moving coil of the electrodynamometer is proportional 
to the product of the magnetic effects of the two coils, and since 
the magnetic effects of the two coils are proportional to the 
current and the voltage, respectively, the indication of the 
instrument must be proportional to the product of (E) and (I), 
or it indicates watts. 




154 



PEACTICAL APPLIED ELECTEICITY 



When an instrument of this kind is connected in a cir- 
cuit in which a fluctuating current, due to a varying load, 
exists, the pressure between lines remaining constant, the 
indications will vary directly with the load current. The 
magnetic effect due to the current in the pressure circuit re- 
mains constant, since the voltage or pressure impressed upon 
this circuit remains constant. When the load current re- 
mains constant and the voltage varies, the magnetic effect 
of the current in the series coil remains constant and the 
magnetic field about the pressure coil changes and the de- 
flection varies with the voltage. 

162. Whitney Wattmeter. — The principle of this wattmeter 
is the same as that of the dynamometer wattmeter just de- 
scribed, except that the construction is modified to make 

the instrument portable and 
more suitable for commer- 
cial work. In this wattmeter 
the heavy current winding 
is composed of two coils, 
which are supported in 
suitable frames and enclose 
a coil composed of fine wire. 
This coil is mounted on a 
shaft with pointed ends rest- 
ing in jeweled bearings. 
Two volute springs serve to 
hold the coil in its zero posi- 
tion and balance the turning 
effort of the coil when there 
is a current in it. These two 
springs also serve to con- 
duct the electricity into and 
from the movable coil, instead of the mercury cups, as in the 
previous case. A balance is obtained by turning the torsion 
head until the needle that is attached to the movable coil is 
indicating no deflection, the reading of the pointer attached to 
the torsion head is then noted and represents the indication of 
the instrument. The scale of the instrument may be divided 
into degrees, and the deflection read on this scale must be 
m^iiltiplied by some constant to obtain the power indicated by 
the instrument, or the scale may be so drawn that the power 




Fig. 114 



ELECTEICAL INSTRUMENTS 155 

in watts can be read directly from it. The general appearance 
of the instrument is shown in Fig. 114. 

163. Weston Wattmeter. — The principle of this instrument 
is practically the same as the one described in the previous 
section. No torsion head, however, is used and the pressure 
coil, instead of being maintained at practically the same 
position for all indications of the instrument, rotates about 
its axis through an angle of approximately eighty degrees. 
Two volute springs serve to conduct the electricity into 
and from the movable coil. They also offer the opposing 
force to that produced by the 

magnetic effect of the cur- 
rents in the two coils. This 
form of instrument has an ad- 
vantage over the other forms 
described in that no manipu- 
lation is necessary and the 
needle points at once to the 
scale mark showing the watts. 
A Weston instrument is 
shown in perspective in Fig. 

115 and diagrammatically in ^^^' ^^^ 

Fig. 116. In Fig. 116 (Ci) and 

(C2) represent the two current coils, which are connected in 
series between the two posts numbered (1) and (2). The 
pressure connections can be made to (3) and (5), or (3) and 
(6). The resistance between (3) and (6) is usually just half 
the value of the resistance between (3) and (5). 

164. Compensated Wattmeter. — The power indicated by 
a wattmeter will be in error just the same as the value of 
the power determined by the voltmeter and ammeter read- 
ings is in error, due to the current taken by the voltmeter, 
or the drop across the ammeter, unless some means be pro- 
vided that will counteract or compensate for this error. This 
compensation is provided in the Weston wattmeter in the 
following way: The pressure circuit is always to be con- 
nected across the line after the current coil, which results 
in the current coil carrying the current that passes through 
the pressure circuit. This current in itself would produce 
a deflection, even though there be no load on the circuit, 
and, as a result, the meter would always indicate a higher 




156 



PRACTICAL APPLIED ELECTRICITY 



value of power than that actually taken by the load.' 
however, the pressure circuit be wound around the series 
coil the same number of times there are turns in the series 
toil, and this winding be so connected that the magnetic 
effect of the current in it is opposite to that of the cur- 
rent in the series coil, the magnetic field produced by the 
pressure current passing through the series coil will be 
completely annulled and there will be no deflection of the 
moving system until there is a load current through the 
series coil. The turns of wire that are wound around the 
Series coil are called compensating turns and they are shown 

in Fig. 116. When a sepa- 
rate source of current and 
potential is used in cali- 
brating wattmeters, there 
should be no compensating 
turns in the circuit, and 
terminal (4) should be 
used instead of terminal 
(3). The resistance of the 
pressure circuit should be 
the same in both cases, 
and a small coil (R), whose 
resistance is equal to that 
oj the compensating turns, 
is connected in the pres- 
sure circuit instead of the 
compensating turns. 

165. Adaptability 
of Wattmeters. — All wattmeters that have been described will , 
operate on either direct- or alternating-current circuits. Their 
indications on direct-current circuits will, however, be in- 
fluenced by stray magnetic flelds, and, in the majority of cases, 
a reversal of current through the coils, although the current 
remains constant in value, will result in a different indication. 
When the instruments are used in an alternating-current cir- 
cuit, these errors are very much reduced — provided the dis- 
turbing fleld is not alternating — since the current passes 
through the circuit in one direction for the same time it passes 
in the opposite direction. 

166. Watt-Hour iVIeters. — The Weston wattmeter, described 




Fig. 116 



ELECTKICAL INSTRUMENTS 



157 



in section (163), is called an indicating wattmeter, since 
it gives the instantaneous value of the watts expended in a 
circuit, just as a voltmeter and ammeter indicate the fluctua- 
tion in voltage and current. The watt-hour consumption in 
any circuit could be determined by means of such a meter 
by multiplying the average indication of the meter for a 



\ left 0md r^MC0iU-''-^ 











^^1 Mm 




Fig. 117 



given time by the time expressed in hours. An integrating 
wattmeter automatically multiplies the average of the in- 
stantaneous readings by the time. The Thomson integrating 
wattmeter, shown in Fig. 117, is similar to the dynamometer 
wattmeter, but the movable coil rotates. It is really a small 
electric motor without any iron in its working parts. The 
revolving part, or armature, is the pressure coil and the 
field in which this coil rotates is produced by the load cur- 
rent in stationary coils mounted outside of the armature. 
The force tending to produce rotation is proportional to the 
product of the two magnetizing effects and this is proper- 



158 PEACTICAL APPLIED ELECTEICITY 

tional to the watts in the circuit, just as in the case of 
the electrodynamometer. The magnetizing effect of the cur- 
rent in the armature is made continuous by winding the 
armature with a number of coils, which are symmetrically 
placed on the armature frame and connected to the external 
circuit through a small commutator and two brushes. The 
current through the armature coils is reversed at such a 
time that the turning effort is always in the same direction. 

The speed of the armature is made proportional to the 
driving force by mounting, on the same shaft that carries 
the armature, a copper disk which is arranged to revolve be- 
tween permanent magnets and constitutes a small generator. 
The torque required to drive this disk is directly propor- 
tional to the speed, and hence, if the torque produced by 
the action of the two magnetizing effects be doubled, the 
speed of the meter will be doubled. The speed of the meter 
at any instant is proportional to the product of (E) and (I), 
or the power. The average speed for a unit of time, say one 
hour, would be proportional to the average power times the 
time it acts (average E X I X time in hours), which gives 
the value of the energy in watt-hours. 

An indicating device is usually attached to the meter, the 
pointers of which are driven by a worm that is rigidly fastened 
to the shaft upon which the armature is mounted. The rela- 
tion between the movement of the pointers on the dials is 
usually such that the dial readings give the energy direct in 
watt-hours, or kilowatt-hours. In some cases, however, the 
dial readings must be multiplied by a constant in order to 
obtain the true watt-hour or kilowatt-hour consumption. 

A diagram of the connections of a Thomson watt-hour 
meter is shown in Fig. 118. The coils (Ci) and (C2) rep- 
resent the series coils connected in series with the line and 
carrying the load current. The pressure circuit, which is 
connected across the line, consists of the resistance coil 
(S) and the winding on the armature (A). The coil (S) 
serves a double purpose: it prevents an excessive current 
passing through the pressure circuit and it creates a mag- 
netic field which is just sufficient to produce the required 
turning effort to overcome the static friction of the meter. 
If this magnetic field were not produced by the coil (S) 
or a similar coil, the armature of the meter would not start 



ELECTEICAL INSTRUMENTS 



159 



to rotate with a small load current in the coila (Ci) and 
(C2), and its indications, as a result, would always be lower 
than the true value of the energy. , The magnetic field pro- 
duced by the coil (S) is in the same direction as that pro- 
duced by the current in the coils (Ci) .and. (C2). The ef- 
fect of this field on the armature can be. varied by changing 
the position of the coil (S) with respect to the armature. 
With this adjustment, the friction of the meter may be prop- 
erly compensated for and it will start to rotate with an 
extremely small current in the series coils. Meters of this 
kind are called integrating wattmeters, since their dials in- 
dicate the average of the product of the instantaneous power 
times the time that power 
acts; or their indication is 
proportional to the energy. 
The watt-hour meter just 
described will operate in 
an alternating-current cir- 
cuit, but not very satis- 
factorily on account of the 
inductance of the w i n d- 
ings. 

167. Coulomb, or Ampere- 
Hour iVleters. — There are 
several recording meters 

on the market whose indications are proportional to the aver- 
age current through them times the time the current exists 
in the circuit, and entirely independent of the voltage produc- 
ing the current. Such instruments are often called coulomb, 
or ampere-hour meters. Meters of this kind can be used in 
measuring energy, when properly calibrated, provided the 
pressure producing the current remains constant. The voltage 
of a line, as a rule, is not always the same, but will fluctuate 
quite a number of volts and, as a result, a coulomb meter will 
not indicate the true energy. 

The old Edison chemical meter was nothing more than a 
coulomb, or ampere-hour meter. It consisted of two zinc 
plates immersed in a solution of zinc sulphate. The quan- 
tity of electricity that had passed through the meter was 
determined by weighing the plates and, from their change in 
weight, the quantity was calculated by means of the elec- 




Fig. 118 



160 



PRACTICAL APPLIED ELECTEICITY 




R 



trochemical equivalent. Fluctuations in line voltage pro 
duced no effect upon the indications of the meter directly; 
voltage changes, however, produce a cnange in the value of 
the current • and this changed the meter's indication. The 
customer's bill was calculated on the assumption that the 
voltage remained constant. 

168. Maximum Demand Meters. — In order to charge the 
purchaser of electrical energy a fair amount for the energy 
consumed in a given time, it is often 
j ^^^ ^ — . desirable to know the maximum 

V/^^A J / ) amount called for during any time, in 

^^^^^\ I y order that he may be properly taxed 

'^^^ ^ ^ or that part of the supply equipment 

that must be held in reserve for him 
so that he may take that maximum de- 
mand at any time. An instrument 
used in measuring the value of this 
maximum deniand is called a max- 
imum demand, or demand, meter. 

The Wright demand meter is shown 

in Fig. 119. It consists of a U-shaped 

tube with enlarged ends and a side 

XV^^y' y tube (S) opening out of one of them. 

^^ ^ A few turns of wire are wound around 

Fig. 119 the enlarged end of the tube (A), that 

has not the side outlet, so that the heat 
generated in the wire due to the passage of a current through 
it will warm the air in the chamber. The U-shaped tube is 
filled with liquid to about the height shown in the figure. The 
expansion of the air in the chamber (A), due to the heat, 
forces the surface of the liquid in the left-hand leg of the tube 
downward and the surface of the liquid in the right-hand 
one upward. If the current strength is sufficiently high to 
cause the liquid in the right-hand tube to rise to the point 
where the outlet tube is connected, the liquid will over- 
flow into the tube (S). The greater the current in the wind- 
ing about (A), the greater the overflow from the tube (R) 
into (S), and a scale properly graduated placed alongside 
the tube (S) will show the maximum amperage that has 
passed through the winding about (A). The indication of 
the instrument is rather slow; that is, it takes some time 






-W- 



ELECTKICAL INSTRUMENTS 



161 



for the liquid in (R) to rise to the point where it will o\^er- 
flow into (S), even though the proper current is passing 
through the winding about (A). This prevents the customer 
being penalized due to a momentary short- 
circuit on his line or the large current 
taken for a short time in starting motors. 
The meter is reset, usually when the con- 
sumer's integrating meter is read, by sim- 
ply inverting the tube and draining all of 
the liquid out of the tube (S) back into 
the U-shaped tube. Changes in the 
temperature of the atmosphere do not in- 
terfere to any great extent with the correct 
registration of the meter, and it will work 
equally well in either a direct- or an 
alternating-current circuit. The whole 
instrument is placed in a case that can 
be locked or sealed, as shown in Fig. 120, 
so that it cannot be reset by a party not 
authorized to do so. 

CALIBRATION OP INSTRUMENTS 



169. Calibration of Voltmeters by 
IVieans of a Standard Instrument. — The 
accuracy of a voltmeter's indications may 
be determined by connecting it in parallel 
with another voltmeter whose calibration 
is known (called a standard), and compar- 
ing their indications for various impressed 
voltages. The connections of the instru- 
ments are shown in Fig. 121, where (S) represents the 
standard instrument, (X) the instrument to be calibrated, 
and (R) a suitable rheostat for varying the pressure over 
the voltmeter terminals. 

170. Calibration of Voltmeters by Means of a Potentiome- 
ter. — The potentiator is a special arrangement for meas- 
uring an electrical pressure by comparison. It is shown 
diagrammatically in Fig. 122. The pressure to be meas- 
ured is connected between the terminals (A — ) and (A-f ). 
There is a variable known resistance connected between 




Fig. 120 



162 



PKACTICAL APPLIED ELECTKICITY 




Fig. 121 



these two points. Across a portion of this resistance, such as 
that between the points (1) and (2), there is connected a 
shunt circuit which consists of a sensitive galvanometer 
(G), a standard cell (SC), a contact key (K), and a high 
resistance (H) that may be easily cut in or out of the circuit. 

The operation of the po- 
c tentiometer is as follows: 

First, assume it is desired 
to know the value of some 
pressure that is connected 
to the points (A — ) and 
(A+). The resistance (R) 
between the points (1) and 
(2) should be adjusted to 
1000 (this need not always 
be 1000) times the voltage 
of the standard cell, which 
is 1.434 for a Clark cell 
at 15° Centrigrade, and 
1.018 30 for a Weston nor- 
mal cell. Assuming a Weston cell is used, then the value of 
the resistance in (R) will be 1018.30 ohms. Now, there will be 
a difference in pressure between the points (1) and (2), 
due to the current in the 
resistance (R) produced by 
the pressure impressed 
upon the terminals (A — ) 
and (A+). The current 
through the resistance (R) 
is in such a direction that 
the point (1) is at a higher 
potential than the point 
(2). This difference in 
pressure between the 
points (1) and (2) would 

tend to produce a current in the shunt circuit when the key 
(K) was closed. The standard cell, however, is connected in 
the circuit in such a way that its electromotive force tends to 
counteract the difference in pressure between the points (1) 
and (2) and there will be no current in the shunt circuit when 
the drop in potential over (R) is exactly equal to the e.m.f. of 




ELECTEICAL INSTRUMENTS 



163 



the standard cell. The drop over the resistance (R) can be 
varied by changing the value of the total resistance between 
(A — ) and (A+). There will be no deflection of the galvan- 
ometer when a balance is obtained. The high resistance (H) 
should be cut out of circuit for a final adjustment. The pur- 
pose of this resistance is to protect the standard cell from 
supplying too large a current while adjusting the drop over 
(R), which is likely to change the value of its e.m.f. and per- 
haps ruin the cell. When this balance is secured, there will be 
1.018 30 volts drop in pressure over 1018.30 ohms in the main 
part of the circuit, or 1 volt over every 1000 ohijas. The total 
difference in pressure between (A — ) and (A+) will then 
be equal to the resistance between (A — ) and (A+) divided 
by 1000. 

Ordinary resistance boxes may be used for the resistances 
shown in the scheme, but commercial forms of the poten- 
tiometer are constructed with all the keys, resistances, etc., 
contained in a single case. 

171. Calibration of Ammeters by Means of a Standard In- 
strument. — An ammeter may be calibrated by connecting it 
in series with a standard 

instrument and comparing 
their indications for differ- 
ent values of current. The 
scheme of connections is 
shown in Fig. 123, in which 
(S) represents the stand- 
ard instrument, (X) the in- 
strument to be calibrated, 
(B) a storage battery or 
some source of electrical 
energy, and (R) a rheostat 

suitable for changing the value of the current in the circuit by 
a change in its resistance. 

A standard resistance may be connected in series with an 
ammeter and the current in the circuit determined by divid- 
ing the drop over the resistance — which may be determined 
by means of a voltmeter or a potentiometer — by the resist- 
ance of the standard resistance. 

172. Calibration of a Wattmeter by Means of a Voltmeter 
and an Ammeter. — The connections for calibrating a watt- 



^ |-| |-| HVWWVWW 




Fig. 123 



164 



PRACTICAL APPLIED ELECTRICITY 



meter or a watt-hour meter by means of a voltmeter 
an ammeter are shown in Fig. 124. The current in the 
series coil of the wattmeter exceeds that in the ammeter by 
an amount equal to the current in the voltmeter. The true 
power, then, should be calculated by adding to the ammeter 
reading (L) the voltmeter current (Iv) and multiplying the 
sum by the voltmeter reading (E), or 

W=(Ia-fIv)E (97) 

The per cent error introduced by not taking the voltmeter 



Pressure Coil 
5eries Coils 




Loa d 



Fig. 124 



current into account is very small, except in the calibra- 
tion of low reading wattmeters. 

When a separate source of current and pressure are used, 
the current taken by the voltmeter produces no error and 
the true power is given by the equation 

W = EXI (98) 

The indication of the dial on a watt-hour meter can be 
checked by maintaining the e.m.f. and current constant for 
a given time and noting the difference in the dial read- 
ings before and after the test. The energy indicated on 
the dial should equal the product of (E), (I), and (t), or 

k.w. hours = E X I X t (99) 

The time (t) in the above equation should be in hours. The 
dial indication can be accurately determined from the num- 



ELECTRICAL INSTRUMENTS 



165 



ber of revolutions of the armature, if the number of revolu- 
tions of the armature per unit indication on the dial is known. 
This relation is called the gear ratio, and once determined 
it can be used in succeeding tests. 

173. Calibration of Wattmeters by Means of Standard 
Wattmeter. — Two wattmeters are shown connected in the 
same circuit in Fig. 125. The instrument (S) is a standard, 




X 



nl 



5 



Load 






Fig. 125 

or one whose calibration is known, while (X) is an instru- 
ment to be calibrated. If the series coils of the two instru- 
ments have the same resistance, the pressure impressed upon 
the pressure circuits of the two meters is the same, when the 
connections are made, as shown in the figure. The same load 
current exists in the series coils of both instruments and the 
indication of (X) can, therefore, be determined in terms of 
the indication of (S). 



CHAPTEK IX 

THE DIRECT-CURRENT GENERATOR 

174. The Dynamo. — The dynamo is a machine for convert- 
ing mechanical energy into electrical energy, or electrical en- 
ergy into mechanical energy by means of electromagnetic in- 
duction, the principles of which were discovered by Faraday 
about 1831. The dynamo, when used to transform mechan- 
ical energy into electrical energy, is called a generator, and 
when used to transform electrical energy into mechanical, it 
is called a motor. The generator does not create electricity, 
but simply imparts energy to it, just as energy is imparted 
to the electricity as it passes through the primary cell. The 
e.m.f. which is generated in the dynamo produces a current 
in a closed circuit connected to the machine and this cur- 
rent will pass from a higher to a lower potential in the ex- 
ternal circuit and from a lower to a higher potential in the 
Internal circuit of the machine itself. This corresponds 
to the flow of water through a pipe connected to a pump. 
In the external circuit, the water flows from a higher to a 
lower pressure, while the action of the pump causes it to 
flow from a lower to a higher pressure through the pump 
itself. 

The dynamo consists fundamentally of two parts — a mag- 
netic field, which may be produced by permanent magnets or 
electromagnets, and an armature, which consists of a coil or 
number of coils usually wound or mounted on an iron core 
and so arranged that they may be revolved or moved in the 
magnetic field of the machine. The movement of the coils 
in the magnetic field results in the conductor forming the 
ceils cutting the magnetic lines of the field and an e.m.f. is 
induced in the winding. 

175. Kinds of Generators. — Generators may be grouped into 
two main classes, depending upon the nature of their output, 
viz., direct-current and alternating-current. 

166 



THE DIEECT-CUERENT GENEEATOE 167 

The current in a circuit connected to a direct-current gen- 
erator is in the same direction all the time, while in the case 
of the alternating-current generator it is continuously revers- 
ing in direction. This chapter will deal with the direct- 
current machine^ the discussion of the alternator being de- 
ferred to a later chapter. 

The mechanical construction of dynamos permits their be- 
ing divided into three classes, the fundamental electrical prin- 
ciple being the same in each case. 

(a) Revolving armature and stationary field magnet. 

(b) Revolving field and stationary armature. 

(c) Armature and field winding both statipnary, but con- 
structed so that an iron core may be revolved near them. 

The external circuit is connected to the armature winding 
in the first class by means of collector rings, see section (177), 
or by means of a commutator, see section (178), while the con- 
nection to the field winding is permanent since it is stationary. 
In the second class the armature is permanently connected 
to the external circuit, and the connection is made to the 
field winding by means of slip rings. There are no moving 
conductors in the third class, but the magnetic flux through 
the armature winding that is produced by the current in the 
field winding is caused to change in value by changing the 
position of a part of the magnetic circuit. This part of the 
magnetic circuit is called the inductor and it is arranged so 
that it may be rotated near the armature and field windings. 

Direct-current generators may also be divided according to 
their output, viz., 

(a) Constant-Current Generators. 

(b) Constant-Potential Generators. 

The output of the constant-current generator is at a con- 
stant current and variable pressure, while the output of the 
constant-potential generator is at a constant potential and 
variable current. 

176. Simple Dynamo. — If a single loop of wire, such as 
(ABCD), Fig. 126, be revolved about an axis, such as (EF), 
in the magnetic field of a permanent magnet, or an electro- 
magnet, as shown in the figure, there will be an electromotive 
force induced in the two sides of the loop (AB) and (CD). 
This induced electromotive force will produce a current in 
the loop if the conductor forming the loop be closed. The 



168 



PEACTICAL APPLIED ELECTEICITY 



direction of the induced e.m.f. in the two sides of the loop 
may be determined by a simple application of Fleming's dy- 
namo rule, as given in section (118). The motion of one side 
of the loop, such as (AB), with respect to the field is just the 
reverse of the motion of the other side (CD). As a result 
of this difference in motion of the two sides of the loop with 
respect to the magnetic field, the e.m.f. induced in one side 
of the loop will be from the observer, while that induced in 




Pig. 126 




the other side will be toward the observer. These two e.m.f.'s 
are acting in series and, since their directions are opposite 
with respect to the position of the observer, they both tend to 
produce a current in the same direction around the loop. The 
value of the current in the loop will depend upon the induced 
e.m.f. and the ohmic resistance of the loop. There will be 
no induced e.m.f. in the ends of the loop since they do not 
cut any of the magnetic lines. 

177. Simple Alternator. — If the loop, shown in Fig. 126, be 
cut at one end and the two ends thus formed be connected 
to two metallic rings (R^) and (R2) mounted on the shaft 
and insulated from each other, as shown in Fig. 127, the com- 
bination will constitute what is called a simple alternator. 
An external circuit can be connected to the coil or armature 
by means of two brushes (Bj) and (B2) that rub on the two 
metallic rings. ' The e.m.f. induced in the armature will now 
produce a current through the external circuit and the armature 
in series. The value of this current will, as in the previous 
case, depend upon the value of the induced e.m.f. and the 
ohmic resistance of the entire circuit. 

The e.m.f. induced in the two sides of the loop at any time 



THE DIKECT-CUERENT GENERATOR 



169 



will depend upon the number of magnetic lines cut in one 
second, or the rate at which the lines are being cut. This 
rate of cutting of magnetic lines will depend upon the length 
of the two sides of the loop, the strength of the magnetic 
field, and the direction in which the conductor is moving with 
respect to the magnetic field. Assuming the strength of the 
magnetic field is uniform and it remains constant, and the 
loop revolves about its axis at a constant rate, then the e.m.f. 
induced in the loop will change in value, due only to a change 
in the direction of motion of the two sides of the loop with 
respect to the magnetic field. Thus, when the loop is in the 





Fig. 128 



Fig. 129 



horizontal position,* the direction of the field also being 
horizontal, the two sides of the loop will be moving in a 
path, just for an instant, perpendicular to the direction of 
the field, and the number of lines of force that the two sides 
of the loop will cut, due to a given motion of the coil, will 
be a maximum. When the loop is in a vertical position, or 
perpendicular to the direction of the magnetic field, any small 
movement of the loop will result in no lines of force being 
cut by the two sides and, hence, no e.m.f. will be induced. The 
value of the e.m.f. induced in the two sides of the loop for- 
positions between those just given, will depend upon the part, 
or component, of the motion that is perpendicular to the 
direction of the magnetic field. This component is propor- 
tional to the sine of the angle between the position of the 
coil and the plane perpendicular to the magnetic field. The 
sine of an angle (6), see Fig. 128, is equal to the length of the 



170 PEACTICAL APPLIED ELECTEICITY 

line (a) divided by the length of the line (b). This relation 
remains the same regardless of the size of the triangle so 
long as the angle (9) does not change in value. From the 
table of ''Trigonometrical Functions" in Chapter 20, may be 
found the values of the sines of different angles from 0° 
to 90°. 

A curve can be drawn which will show graphically the rela- 
tion between the induced e.m.f. in the coil and its angular 
position with respect to some reference plane, say the 
plane parallel to the magnetic field. Draw a line (AB), 
Fig. 129, and divide this line into, say, 90 equal parts, each 
part will then correspond to four-degrees movement of the 
coil about its axis. Starting with the coil in a plane per- 
pendicular to the magnetic field, and let this correspond to the 
point (A) in the figure, the e.m.f. induced in the coil for any 
angular displacement from this position should be measured 
off to a convenient scale on a line drawn through the point 
on (AB), corresponding to the displacement of the coil. Thus, 
the e.m.f. will be a maximum when the coil has rotated 
through an angle of 90°, since the value of the sine of the 
angular displacement is increasing up to this point. It then 
decreases as the angle increases from 90° to 180° and becomes 
equal to zero when the coil has rotated through 180°. The 
direction of the movement of the two sides of the loop with 
respect to the magnetic field changes just as the coil passes 
through its 180° position and, as a result, the direction of the 
induced e.m.f. changes. The numerical values of the e.m.f. 
for the second 180° are identical to those for the first 180°, 
but they are opposite in sign. The difference in sign is 
represented in the curve by drawing the second part of the 
curve below the horizontal line. 

178. Simple Direct-Current Dynamo. — Purpose of the Com- 
mutator. — The e.m.f. induced in the loop of wire described 
in the previous section can be made to produce a direct cur- 
rent — one that is constant in direction in the external circuit — 
in the following way: Suppose the two continuous metallic 
rings be replaced by a single ring composed of two parts that 
are insulated from each other, the distance between the ends 
of the two parts composing the rings being small in compari- 
son to the total circumference of the combined ring. If the 
two ends of the loop be connected to these two parts of the 



I 



THE DIEECT-CUKEENT GENEEATOE 



171 



ring, which we shall call segments, and two brushes that 
are insulated from each other be so mounted with respect to 
each other and the ring that they rest upon the insulation 
between the segments when the e. m. f. induced in the coil is 
zero, the connection of the external circuit with respect to 
the loop will be reversed at the same instant the direction of 
the induced e.m.f. in the loop changes. This results in the 
induced e.m.f. in the coil always tending to send a current 
through the external circuit in the same direction. The proper 
arrangement of the loop, segments, and the brushes is shown 
in Fig. 130. Such a machine is called a simple direct-current 
dynamo, because it delivers a direct current to the external 
circuit. The two-part ring constitutes a simple commutator 
of two segments, and its purpose, as pointed out, is to reverse 
the connection of the external circuit with respect to the 




Fig. 131 



armature winding, or vice versa, so that the induced e.m.f. 
in the winding will send a direct current through the external 
circuit. A curve representing the value of the e.m.f. impressed 
upon the external circuit when the two-part commutator is 
used is shown in Fig. 131. An e.m.f. such as that represented 
in Fig. 131 is called a pulsating e.m.f., because it pulsates or 
changes from zero 'to a maximum and back to zero at regular 
intervals. 

179. Multiple-Coil Armatures. — If the armature of a direct- 
current generator were constructed with a single coil com- 
posed of one or more turns, the current delivered by such a 
machine would pulsate in value the same as the e.m.f., as 



172 



PEACTICAL APPLIED ELECTKICITY 




Fig. 132 



shown in Fig. 131. The operation of such a machine would 
be very unsatisfactory in a great many cases. Fortunately the 
e.m.f. between the brushes of the machine can be made more 
nearly constant in value in the following way: 

Suppose two closed coils 
A o (A) and (B) be placed on 

the armature at right an- 
gles to each other, then 
the induced e.m.f. in one 
of them will be a maxi- 
mum when the induced 
e.m.f. in the other one is 
zero. Now as the arma- 
ture rotates from such a 
position that the plane of 
coil (A) is perpendicular 
to the magnetic field — zero 
e.m.f. induced in it — and 
the plane of coil (B) is 
parallel to the magnetic field — maximum induced e.m.f. in 
it — the induced e.m.f. in coil (A) will increase and the 
induced e.m.f. in coil (B) will decrease, and this action will 
continue until the armature has turned through an angle 
of 90°. After the armature has turned through an angle of 
90°, the e.m.f. in coil (A) is a maximum and that in coil (B) ' 
is zero. For the next 90°, the e.m.f. in coil (A) is decreasing 
and the e.m.f. in coil (B) is increasing, but in the opposite di- 
rection. The corresponding values of induced e.m.f. will occur 
in the two coils at inter- 
vals which differ in value 
by one-fourth revolution, 
or. 90°. The two curves (A) 
and (B) in Fig. 132 show 
the relation of the e.m.f.'s 
in the two coils. 

If now a four-segment 
commutator be used and 
two coils be symmetrically 

placed on the armature, the terminals of each coil being con- 
nected to opposite commutator segments, the e.m.f. between 
the brushes of the machine will never fall to zero value but 



90^ 



180 
Fig. 133 



no' 360 



THE DIEECT-CUEEENT GENEEATOE 



173 



will be of the iorm shown by the full line in Fig. 133, when 
the brushes are properly placed with respect to the commu- 
tator. The e.m.f. can be made more nearly constant by 
placing more coils on the armature in such a position with 
respect to the first ones that the induced e.m.f. in them does 
not reach a maximum or zero value at the same time it does 
in the others, and connecting them in such a way that the 
induced e.m.f. in all of the coils acts in series and in the 
same direction as far as the external circuit is concerned. 

180. The Armature of a Dynamo. — The loops of wire, or 
coils, in which the e.m.f. is induced by a movement of them 
with respect to the magnetic field, together with the iron parts 
upon which they are mounted, insulating material, and slip 
rings or commutator — in the cases where the coils move — 
constitute what is called the armature of a dynamo. The classi- 
fication of armatures, their construction, and methods of wind- 
ing will be taken up in detail in the chapter on ''Armatures 
for Direct-Current Dynamos." 

181. Magnetic Field of a 
Dynamo. — In the majority of 
cases the magnetic field of 
a dynamo is produced by 
electromagnets. In the case 
of magnetos, however, the 
magnetic field is created by 
several powerful permanent 
horseshoe magnets. Such a 
machine is shown in Fig. 
134. Small machines are 
usually bipolar, that is, they 
have one N-pole and one 

S-pole which create the magnetic field in which the armature 
rotates. These magnetic fields assume a number of different 
forms, a few of which are shown in Fig. 135. 

In large machines it is customary to use multipolar field 
magnets in which any even number of poles are arranged 
alternately around the armature. The magnetic circuit of a 
machine whose field is created by an electromagnet usually 
consists of five parts, as shown in Fig. 136. (1) The field 
cores (C) are the centers of the coils carrying the magnetiz- 
ing current. (2) The yoke (Y) connects the field cores 




Fig. 134 



174 



PKACTICAL APPLIED ELECTRICITY 



together at one end, as shown in the figure. (In some 
machines there is no yoke in the magnetic circuit, see Fig. 
135 A.) (3) The pole pieces (P) are the metallic parts of the 
magnetic circuit next to the armature. They are usually cut 
to conform to the curvature of the armature and in some 
cases are an entirely different piece of metal than the field 
cores, being fastened to the end of the field cores by means 
of bolts. The surface of the pole pieces next to the armature 



c 


p()p 


\ 


^^^\ 










"^^m^ 






Fig. 135 



is called the pole face; and the projecting edges, when so 
constructed, are called the horns. (4) The air gap (G) is the 
space between the pole face and the armature core. (5) The 
armature core conducts the magnetic lines from one air gap 
to another. 

The coils carrying the magnetizing current may be placed 
on the magnetic circuit as shown in Fig. 135 D or they may 
be placed on the magnetic circuit as shown in Fig. 135 C. 
When the field windings are placed, as shown in Fig. 135 D, 
the magnetomotive force created by the current in one wind- 



THE DIRECT-CUKRENT GENERATOR 



175 



ing is in series with that created in the other winding, or the 
magnetomotive force on any magnetic circuit is that produced 
by two windings in series. If the windings he placed upon 
the magnetic circuit, as shown in Fig. 135 C, the magneto- 
motive force acting on each magnetic circuit would be that 
of a single coil. This results in only half the ampere-turns 
per coil being required when they are placed as shown in 
Fig. 135 D, as would be required if the coils were placed as 
shown in Fig.sl35 C. 

182. Materials Used in the Construction of the Magnetic 
Circuit.-— There are three materials that are commonly used 

in the construction of the 
magnetic circuit of a dyna- 
mo — cast iron, wrouglit 
iron, and cast steel. There 
are a number of factors 
which govern the selection 
of the materials to be used 
in a particular machine, 
such as initial cost, weight, 
efficiency demanded b y 
purchaser, regulation, etc. 
The cheapest of the 
above materials is cast 
iron and it is the poorest 
of them all magnetically. 
So the saving in the initial cost of the iron per pound would 
more than likely be offset by the fact that a larger bulk of cast 
iron would be required to form a certain magnetic circuit than 
would be required if wrought iron, for example, were used. 
There would also be an increase in the cost of copper required 
to magnetize the large cast-iron core, since the length of each 
turn would, be more than when some better magnetic mat(^rial 
was used. 

Wrought iron, on the other hand, is the best magnetic 
material and at the same time the most expensive. It is used 
where economy in weight and reduction of cross-section are 
desired. Machines used aboard ships or electric automobiles, 
etc., are usually made of wrought iron on account of the large 
reduction in weight, which is a more important factor than 
the initial cost. 




Fig. 136 



176 PEACTICAL APPLIED ELECTRICITY 

Cast steel occupies a place intermediate between cast iron, 
and wrought iron both in cost and magnetic properties. I 
Machines are, as a rule, constructed of more than one mate- 
rial. Thus the field cores will be of wrought iron, as that 
would mean a saving in copper since the length of wire per 
turn would be less than if cast iron were used; the yoke may- 
be of cast iron as its area can be made much larger than that / 
of the field cores and this increase in area will add strength 
to the machine. The armature core is usually constructed of 
wrought-iron sheets, so as to reduce the eddy-current loss to 
a minimum; the pole shoes may be cast or they are sometimes 
laminated in order to reduce the eddy-current loss in them. 

183. Magnetic Leakage, Shape of Magnetic Circuit. — The 
total number of magnetic lines created by the current in the 
field winding do not pass through the armature core, and 
therefore, they are not all useful in inducing an e.m.f. in the 
armature winding. The ratio of the total number of magnetic 
lines created to the number that are actually useful in gen- 
erating an e.m.f. is called the leakage coefficient. The value 
of this coefficient can never be less than unity and in the 
case of poorly designed machines, it will reach a value as 
high as 1.4. 

The value of this coefficient can be reduced by constructing 
the magnetic circuit so it will be as short as possible, or of 
low reluctance, and without abrupt turns. Placing the field 
winding upon or near that part of the magnetic circuit offer- 
ing the greatest reluctance will also reduce the value of the 
leakage coefficient. 

184. Value of Induced E.M.F. in Armature Winding. — The 
induced e.m.f. in an armature winding depends upon the 
following factors: 

(a) The useful fiux ($) per pole of the machine. 

(b) The number of poles (p) composing the magnetic 

circuit. 

(c) The total number of inductors (Z) on the armature. 

(d) The number of paths (b) in parallel through the arma- 

ture. 

(e) The speed (s) in revolutions per second. 

(f) The number of magnetic lines (108) that must be cut 

per second in order that there be an e.m.f. of one 
volt induced. 



THE mRECT-CURRENT GENEEATOR 177 

These factors can all be combined in a simple equation which 
will give the value of (E) in volts between the brushes of 
the machine when it is operating without load, or when it is 
delivering no current. 

* X P X Z X s 

E = ^ (100) 

108 X b 
The value of (b) in the above equation will depend upon the 
type of winding on the armature. For a simple lap winding 
(singly re-entrant) it is equal to the number of poles and lor 
a simple wave winding (singly re-entrant) it is always two 
regardless of the number of poles. This will be taken up 
again in the chapter on ''Armature Winding." 

Example. — A six-pole generator is operating at 900 revolu- 
tions per minute. The useful flux per pole is 4 000 000 lines. 
The armature has a simple lap winding (singly re-entrant) 
of 198 inductors. What e.m.f. will be generated? 

Solution. — First obtain the value of the speed in revolutions 
per second by dividing 900 by 60, or 

900-^60 = 15 
The machine is then running at 15 revolutions per second. 
Since the winding on the armature is a simple lap winding, 
.the value to substitute for (b) in equation (100) is equal to 
the number of poles, or 6. Substituting the values of the 
various quantities in equation (100) gives 

4 000 000 X 6 X 198 X 15 

E == = 118.8 

100 000 000 X 6 

Ans. 118.8 volts. 
185. Separate Excitation and Self-Excitation of a Genera- 
tor. — There are two methods by which the electromagnets of 
the generator may be energized, and they are 

(a) Separate excitation. 

(b) Self-excitation. 

In the case of separate excitation, the field windings of the 
machine are connected to some source of energy, other than 
the dynamo under consideration, such as a storage battery 
or another dynamo, and the currept in the field winding is 
independent of the e.m.f. generated by the machine that is 



178 



PEACTICAL APPLIED ELECTRICITY 



le Dy 



being excited. The field current is adjusted in value Dy 
changing the value of the resistance in the field circuit, which 
is accomplished by what is known as a field rheostat, or by 
changing the e.m.f. of the generator that is supplying the 
current to the field. The connections for separate excitation 
are shown diagrammatically in Fig. 137. The rheostat (Ri) 
is connected in series with the field of the main generator 




Fig. 137 

(G). The field of the main generator may be connected to 
either the storage battery (B) or the small generator (E), 
which is called the exciter. A rheostat (R2) is connected in 
series with the field of the generator (E) and by means of this 
rheostat the voltage generated in the armature of the exciter 
can be changed, and hence the current in the field of the 
generator (G), if (E) is supplying current to the field. 

There are three different methods of self-excitation and 
they will each be taken up in turn in the following three 
sections. 

186. Shunt -Wound Generator. — 
The field v/inding in the case of a 
shunt-wound generator consists of a 
large number of turns of relatively 
I "V^ P T'®!^ small wire that may be connected 

I — L JW in ding directly across the terminals of the 

Fig. 138 machine. The potential difference 

between the two terminals of the 
machines produces a current in the field windings, which is 
regulated in value by a resistance or rheostat. A diagram of 
the connections of a self-excited shunt machine is given in 
Fig. 138. 

187. Series-Wound Generator. — The field winding in the 
case of a series-wound generator consists of a few turns of 
large wire connected directly in series with the armature and 




\ R 
Shunt 
Field 
Winding 



THE DIKECT-CURRENT GENERATOR 



179 



load. The current in the field windings of a machine of this 
kind is of the same value as the current through the load. A 
diagram of the connections of a series-wound machine is 
given in Fig. 139. 





Fig. 139 



Fig. 140 



188. Compound-Wound Generator- — In the case of a com- 
pound-wound generator, the field windings consist of two sets 
of coils. The coils of one set consist of a few turns of large 
wire connected in series with the armature and the load; the 
coils of the other set consist of a large number of turns of 





Pig. 141 



Fig. 142 



small wire, all connected in series across the terminals of the 
machine. The compound generator is nothing more than a 
combination of a shunt and series generator. 

When the shunt winding is connected directly across the 
armature, as shown in Fig. 140, the machine is called a short 
shunt; when it is connected across the armature and the 
series field, as shown in Fig. 141, the machine is called a long 



180 PKACTICAL APPLIED ELECTEICITY 

shunt. In the case of a long shunt the current in the sSunf 
field passes through the series field in completing its circuit 
through the armature; while in the case of a short shunt, the 
current in the shunt field passes directly through the arma- 
ture without passing through the series field. The current in 
the shunt field can be regulated by a rheostat (R), while the 
current in the series field depends upon the current in the 
load to which the machine is connected. 

189. Armature Reaction. — When a dynamo is operating 
under a load, the current in the armature will produce a 
magnetizing effect upon the field of the machine and this 
effect is called armature reaction. The direction of this mag- 
netizing effect due to the armature current can be determined 
as follows: Take a simple drum armature with twelve coils 
equally spaced around the core and revolving in a bipolar 
magnetic field. (A cross-section through such an armature 
and field is given in Fig. 142.) The induced e.m.f. in the 
armature inductors on the right of a plane (AC), drawn 
perpendicular to the direction of the magnetic field, will be 
just the reverse of the direction of the induced e.m.f. in the 
conductors to the left of the plane (AC). The induced e.m.f. 
in the conductors is zero when they are in the plane (AC), and 
it changes in direction as the conductors pass from one side 
of the plane to the other. The plane (AC) is called the 
normal neutral plane, it being perpendicular to the magnetic 
flux when there is no current in the armature and, as a result, 
the field is not distorted. The brushes (B^) and (B2) should 
be placed in this plane, and when they are connected through 
an external circuit there will be a current in the armature 
conductors the value of which will depend upon the value of 
the induced e.m.f. and the total resistance of the circuit. The 
direction of this current in the inductors will be the same as 
that of the induced e.m.f. shown in Fig. 142, and it will 
produce a certain magnetizing effect upon the armature core. 
This magnetizing effect can best be shown by connecting the 
terminals of the machine to some source of e.m.f. in such a 
way that the current in the armature will be in the same 
direction as though it was produced by the induced e.m.f. 
(When the machine is connected to the external source of 
e.m.f., there should be no current in the field windings and 
the armature should be stationary.) The magnetic field due 



THE DIRECT-CURRENT GENERATOR 



181 




Fig. 143 



to the armature current alone is shown in Fig. 143. It is 
seen that this field is at right angles to the field of the 
machine. In actual operation the magnetizing effect of the 
armature current and the 
field current are both 
present at the same time 
and the resultant field is 
a combination of the two 
as shown in Fig. 144. 
This results in a non- 
uniform distribution of 
the magnetic flux through 
the generator pole pieces, 
air gaps, and armature 
core. The distortion 
takes place in the direc- 
tion of rotation which re- 
sults in a crowding, of 
the fiux in the trailing 
horns of the pole pieces. 

The neutral plane of the resultant magnetic field will make 
an angle with the normal neutral plane, and the value of this 
angle will depend upon the amount of armature reaction. As 
a result of the neutral plane changing its position, the posi- 
tion of the brushes must also be changed so that it 
will correspond more nearly to the position of the 
neutral plane. ' With a change in the position of the brushes, 
there will be a change in the direction of the current in some 
of the armature conductors. Thus, if the brushes be shifted 
in the direction of rotation, as shown in Fig. 145, the direction 
of the current in the inductors in the angle (O) will change 
and the magnetic effect of a current in the armature will no 
longer be in a direction perpendicular to the magnetic field 
of the machine, but in a direction such as that shown in 
Fig. 145. This magnetizing effect can be thought of as made 
up of two parts, one acting parallel to the magnetic field of 
the machine and another acting perpendicular to the magnetic 
field of the machine. The magnetizing effect of the armature 
current perpendicular to the field of the machine is called the 
cross-magnetizing effect, and the effect parallel to the field of 
the machine is called the demagnetizing effect. One of the 



182 



PEACTICAL APPLIED ELECTKICITY 




above effects tends to weaken the magnetic field of the 

machine, while the other tends to distort the magnetic field. 

190. Cross-Turns and Back Turns. — In the previous section 

it was pointed out that 
y\ the plane of the brushes, 

)/B, known as the commutat- 

J:t^ ing plane, should be moved 

"^/?iX- : 1'. - : -^ in the direction of rota- 
^^V.V-I-J tion in order that the 
- v-^--\---''] brushes be nearer the neu- 
'4'Kl--S'< ^^^^ plane. The position 
'^"^l'\ ^^ ^^^^ commutating plane 
^jl'^yjl will always be in advance 
^ —J of the normal neutral 

iQt)^\ ^- 1 plane in the case of a 

B rj-'^ generator, and behind the 

1 normal neutral plane in 

Q the case of a motor. 

Fig. 144 The angle between the 

commutating plane and 
the normal neutral plane is called the angle of lead in the case 
of a generator, and the angle of lag in the case of a motor. 
The position of the two commutating planes is shown in 
Fig. 146, the full line (DE) 
representing the commu- A 

tating plane of the gener- 
ator and the dotted line 
(FG) representing the 
commutating plane of the 
motor. The conductors in 
the double angle (29) on 
one side of the armature 
can be thought of as being 
in series with the con- 
ductors in the angle (20) 
on the other side of the 
armature, and forming a 
number of complete turns 
about the core. The re- 
maining conductors can be thought of as forming a second set 
of turns. The product of the turns in the double angle (29) 




THE DIRECT-CUREENT GENERATOR 



183 




and the current in them gives what is called the demagnetizing 
ampere-turns because their effect is to produce a weakening of 
the magnetic field of the machine. The remaining turns times 
the current in them are called the cross-magnetizing ampere- 
turns because they act at 
right angles . to the mag- 
netic field of the machines. 
The turns in the angle 
(29) are called back 
turns and the remaining 
ones are called cross-turns. 
191. Means of Reduc- 
ing Armature Reaction. — 
Armature reaction inter- 
feres with the satisfactory 
operation of the dynamo 
and should be reduced to a 
minimum where possible. 
There are a number of 
ways of bringing about a 

reduction in the effect of armature reaction, some of the more 
important ones being: 

(a) By increasing the length of the air gap of the machine. 

(b) By slotting the poles parallel to the axis of the arma- 

ture. 

(c) By properly shaping the pole pieces. 

(d) By using auxiliary poles. 

(e) By placing a winding in perforations in the pole faces. 

(Invented by Mr. Ryan.) 

(a) Increasing the length of the air gap increases the 
reluctance in the magnetic circuit that is acted upon by the 
cross-magnetizing ampere-turns, but at the same time increases 
the number of ampere-turns required in the field winding of 
the machines. Thus the effect of the distorting, or cross-turns, 
will not be so great as it would be if the magnetic field of the 
machine were weaker. 

(b) Cutting slots in the pole faces parallel to the axis of 
the armature introduces a large reluctance in the path of the 
cross flux, produced by the cross-magnetizing ampere-turns, 
but does not introduce anything like as great a reluctance in 
the main magnetic circuit. Fig. 147 shows a stamping used in 



184 



PBACTICAL APPLIED ELECTKICITY 



the construction of a field core, there being a small slot 
punched in each piece. When these stampings are all 
assembled there will be one long slot through the pole piece 
parallel to the axis of the armature. 

(c) The shifting of the magnetic flux across the pole shoe 
of the machine can be greatly reduced by properly shaping 



Fig. 147 Fig. 148 

them so that the parts of the air gap where the flux tends to 
become most dense will have the greater reluctance. Thus the 
pole faces may be cut so that the trailing horn, in the case of 
a generator, will be farther from the surface of the armature. 
The trailing horn of the pole piece may also be made longer 

than the advancing side, 
which also results in a 
more uniform distribu- 
tion of the magnetic flux 
and the armature in- 
ductors will enter and 
leave the magnetic field 
more gradually than they 
would if an ordinary pole 
piece were used. Such a 
pole piece is shown in 
Fig. 148. 

(d) Auxiliary poles 
may be placed between 
the main poles of the ma- 
chines and so connected 
^is. 149 tl^at their magnetizing 

effect is just the reverse of that of the cross-magnetizing am- 
pere-turns. The windings on these poles are connected so that 
they carry all or a definite portion of the full load current. 
This results in their effect varying directly as the load cur- 
rent, just as the effect of the cross ampere-turns varies with 




THE DIRECT-CUERENT GENERATOR 



185 




the load current, and if the effects balance for one particular 
load, they will practically balance for all other loads and the 
position of the neutral plane of the magnetic field will remain 
almost constant. The auxiliary poles of a machine are shown 
in Fig. 149. 

192. Commutation. — The process of commutation can best 
be explained by reference to Fig. 150. The commutator seg- 
ments are shown shaded while the various elements of the 
armature winding are 
shown connected in series, 
the junctions of these ele- 
ments being connected to 
the commutator segments 
in regular order. The direc- 
tion of rotation of the arma- 
ture is indicated by the 
arrow (A), and the position 
of the neutral plane by the 
line (DE). The direction 
of current in the various 
elements is indicated by the 
small arrows. With a direc- 
tion of current corresponding to that shown in the figure, the 
brush (B^) must be positive. 

Now as the armature rotates, the commutator segments in 
turn pass under the brush (Bi). If the contact surface of the 
brush is greater than the width of the insulation between the 
segments, which should always be the case, then an element 
ot the winding will be short-circuited when the brush is in 
contact with the two segments to which the element is con- 
nected. The current in the short-circuited coil drops to zero 
when it is shorted, but it does not do so instantly on account 
of the inductance of the coil. As the armature rotates, the 
thrush moves from commutator segment (C4), Fig. 150, and 
the element of winding (4) is connected in series with 
the other elements in the left-hand path. When the element 
becomes a part of the left-hand path, it carries the same 
c current the other elements in that path carry. Now if there 
is zero current in the coil that is finishing commutation, 
just as it moves from the short-circuited position, the current 
in it must increase almost instantly to a value equal to that 



Fig. 150 



186 PRACTICAL APPLIED ELECTRICITY 

in the other elements. The inductance of the element op- 
poses this sudden increase in current and, as a result, there 
is a tendency for an arc to form between the brush and the 
commutator segment (C4) until the current in the coil (4) has 
reached its proper value, or the inductance of the coil has been 
overcome. This condition of affairs would result in a con- 
tinuous sparking at the brushes, which would not only repre- 
sent a loss but would be injurious to both the commutator and 
the brushes. Sparking due to the cause just mentioned can 
be reduced and practically overcome by advancing the brushes 
beyond the neutral plane. When the brushes are thus ad- 
vanced, there will be an e.m.f. induced in the coil undergoing 
commutation while it is short-circuited and this induced e.m.f. 
will be in such a direction as to produce a current in the same 
direction as the current in the elements to the left of the 
brush, as shown in Fig. 150. This results in the inductance 
of the coil being overcome while the coil is still short-cir- 
cuited, and the current will meet with no opposition other 
than the ohmic resistance when the coil becomes a part of 
the left-hand circuit. Advancing the brushes beyond the 
neutral plane results in a slight lowering of the terminal e.m.f. 
of the machine, but this is more than offset by the advantage 
in the reduction in sparking. 

193. Capacity of a Generator. — The output of a generator is 
limited by one of the three following factors, when sufficient 
power is applied to drive it. These factors are: 

(a) Excessive drop in the armature of the machine, 

(b) Excessive heating. 

(c) Excessive sparking. 

(a) As the load on a generator is increased, there is an 
increase in drop in the armature due to the (IR) drop and 
armature reaction. These two effects combined decrease the 
terminal voltage of the machine and this decrease is usually 
excessive when the machine is overloaded. 

(b) The allowable temperature rise as prescribed by the 
American Institute of Electrical Engineers is as follows: 
*'Under normal conditions of operating and ventilation, the 
maximum temperature rise referred to a standard room tem- 
perature of 25° C. should not exceed 50° C. for field coils and 
armature as measured by the increase in resistance; and 
55° C. for commutator and brushes, and 40° C. for all other 



I 



THE DIRECT-CUEEENT GENERATOR jg? 

parts, bearings, etc., as determined by a thermometer " It is 
usually safe to operate a machine above these temperatures 
for a few hours, but an excessive heating of commutator 
armature, or field coils, is 
likely to injure and in 
some cases destroy the in- 
sulation. 

(c) The heat generated 
as a result of sparking 
usually limits the allow- 
able sparking, as it causes 
a rise in temperature of 
the commutator and 
the brushes. 

194. Building Up of a 
Self-Excited Shunt Gener- 
ator. — The iron composing 




field Current 

Fig. 151 



the magnetic circuit of a generator usually retains some of its 
magnetism and when the armature is revolved in this weak 
magnetic field, there is a small e.m.f. induced which produces 
a current in the field windings. This current, if the windings 

are properly connected, 
will increase the magnetic 
flux through the arma- 
ture, which in turn will 
Increase the e.m.f. and 
field current, etc. A curve 
showing the relation be- 
tween terminal voltage 
and field current is given 
in Fig. 151. Such a curve 
is called a magnetization 
curve. The abrupt bend 
in the curve near the top 
indicates that the mag- 




-oad Current 

Fig. 152 



laetic circuit is practically saturated. 
i 195. External Characteristics of Shunt, Series, and Com- 
ipound Generators.— The external characteristic curve of a gen- 
3rator is a curve that shows the relation between the current 
loutput of the generator and the terminal voltage. 
ti In the case of the shunt generator, assuming the speed 



PEACTICAL APPLIED ELECTKICITY 



[ afteal 



o 

:> 



/ 



188 

remains constant and the field resistance is not changed 
it is adjusted to give normal terminal voltage at no load, the 
voltage at the terminals of the machine will decrease with an 
increase in load on account of armature reaction and copper 

drop. The curve in Fig. 152 
shows the relation between 
the terminal voltage and 
the load current for a shunt 
generator. The drop in 
voltage will be different 
for different machines, de- 
pending upon the resist- 
ance of the armature and 
the amount of armature 
reaction. 

In the case of a series 
generator, the terminal 
voltage is zero with zero 



-C37 

:oi 



.CHI 



Load Current 

Fig. 153 



load but it increases as the load current increases because 
the field excitation is increasing. The field strength of the ma- 
chine will continue to increase very rapidly with an increase 
in load until the iron becomes saturated, when the effects of 
armature reaction and cop- 
per drop produce a de- 
crease in terminal voltage 
with a further increase in 
load, as shown in Fig. L53. 
In the case of a com- 
pound generator, the series 
winding may be so con- 
nected as to produce a 
magnetizing effect that 
aids the shunt field, and 
with an increase in load 
there is an increase m 
total field excitation. If 

this increase in field excitation is just sufficient to main- 
tain the terminal voltage practically constant, the ma- 
chine is said to be flat-compounded. If there is a rise m 
terminal voltage with an increase in load, the machine is 
said to be over-compounded, and if the voltage drops with an 




Load Current 

Fig. 154 



THE DIRECT-CUERENT GENERATOR 



189 



Increase in load the machine is said to be under-compounded. 
The external characteristic curve of an over-compounded gen- 
erator is shown by curve (A) ii. Pig. 154, and the external 
characteristic curve of a flat-compounded generator, by 
curve (B). 

196. Adaptability of Shunt, Series, and Compound Gen- 
erators. — The shunt generator is usually used where it is 
desired to have a practically constant voltage, and the dis- 
tance from the machine to the load is not very great, resulting 
in a small voltage loss in the line. 

The series generator is usually used in supplying a constant 
current to a load at a varying potential, such as a number of 
arc lamps connected in series. 

The compound machine 
can be constructed so that 
the voltage at its ter- 
minals, or at the load, can 
be maintained constant or 
allowed to increase or de- 
crease with a change in 
load. Thus it can operate 
a number of lamps at a 
constant pressure even 
though they be located 
some distance from the 
generator, or the voltage 
at the end of the line can 
be made to increase with 
an increase of load, as is 
quite often the case in 
railway work. A modern compound generator is shown in 
Fig. 155. 

197. Losses in Generators. — The losses in generators may 
be divided into two main groups: 

(A) I2R, or electrical losses. 

(B) Stray-power losses. 

(A) The I2R losses occur in the field windings and the 
armature. If (Ic), (Is), and (la) represent the currents in the 
shunt-field winding, series-field winding, and armature, re- 
spectively, and (Re), (Rs), and (Ra) represent the resistance 
of the shunt-field winding, series-field winding, and armature. 




Fig. 155 



190 PEACTTCAL APPLIED ELECTEICITY 

respectively, then the loss in the shunt-field winding (Wc), 
series-field winding (Ws), and armature (Wa) can be deter- 
mined by the following equations : 

Wc = l2cRc (101) 

Ws = I2sRs (102) 

Wa = I2aRa (103) 

(B) The stray-power losses consist of 

(a) Hysteresis and eddy-current losses chiefly in the arma- 

ture core. 

(b) Friction losses at bearings and brushes, and air fric- 

tion, or windage, as it is called, due to the fan-like 

action of the moving parts. 
The stray-power losses cannot be calculated with the same 
degree of accuracy that the (I^R) losses can; but they can, 
however, be quite accurately determined for a given machine 
by experiment. 

198. Efficiency of Generators^ — There are three efficiencies 
for a generator: 

(a) Efficiency of conversion- 

(b) Electrical efficiency. 

(c) Commercial efficiency. 

(a) The efficiency of conversion is the ratio of the total 
electrical power generated to the total mechanical power 
supplied. Let (P) represent the mechanical power supplied, 
then 

EI + (electrical losses) 

Efficiency of conversion = X 100 

P (104) 

Where (EI) represents the output in watt. 

(b) The electrical efficiency is the ratio of the total elec- 
trical power delivered to the total electrical power developed. 

EI 

Electrical efficiency = X 100 

EI + (electrical losses) (105) 

(c) The commercial efficiency of a generator is the ratio 
of the electrical output to the mechanical input, or 

EI 
Commercial efficiency = — X 100 ' (106) 

P 



THE DIEECT-CUERENT GENEEATOE 191 

The commercial efficiency is the most important of the three, 
as it includes all the losses in the machine. 

199. Commercial Rating of Generators. — Generators are 
rated according to their k.w. output. Thus a 100-k.w. 100-volt 
generator means the machine will deliver 100 k.w. to an 
external circuit connected to its terminals, and that the 
voltage will be 100 volts: If the output is 100 k.w. at 100 
volts, then the current will be 10 000 -^ 100 = 1000 amperes. 



PROBLEMS ON DIRECT-CURRENT GENERATORS 

1. Calculate the e.m.f. generated in -the armature of a 10- 
pole direct-current generator wound with 1000 inductors, lap \ 
winding (simplex singly re-entrant), and revolving at 300 revo- 
lutions per minute. The magnetic flux per pole is 5 000 000 
maxwells. Ans. 250 volts. 

2. If the winding in the above problem was changed to a 
wave winding (simplex singly re-entrant), what e.m.f. would 
be generated? ^ Ans. 1250 volts. 

Note: See section (184). 

3. If the speed in problem (1) is decreased to 250 revolu- 
tions per minute and the flux ($) per pole is raised to 
6.000 000 maxwells, what would be the change in the e.m.f. 
generated? Ans. No change. 

4. The armature of the machine in problem (1) has a 
resistance of .006 ohm, what will be the value of the terminal 
voltage when the machine is delivering a' current of 750 
amperes? (Assume the internal voltage remains constant.) 

Ans. 245.5 volts. 

5. How much should the flux per pole be increased in order 
that the terminal voltage in problem (4) remain constant? 

Ans. Increase 90 000 maxwells. 

6. There are 180 inductors on the surface of a bipolar drum- 
wound armature and in each of these inductors there is a 
current of 50 amperes. Calculate the demagnetizing and cross- 
magnetizing ampere-turns when the commutating plane makes 
an angle (9) of 10 degrees with the normal neutral plane. 

Ans. 500 demagnetizing ampere-turns. 
4000 cross-magnetizing ampere-turns. 



192 



PKACTICAL APPLIED EL.ECTEICITY 



7. The total flux produced by a field winding is 5 800 000 
maxwells and the useful flux is 5 000 000 maxwells, calculate 
the coefficient of magnetic leakage. 

Ans. Leakage coefficient = 1.16. 

8. The output of a 110-volt generator is 300 amperes, what 
is the horse-power input if the eflaciency of the machine is 
90 per cent? Ans. 49.2 horse-power. 

9. If the electrical loss in problem (8) is 1375 watts, what 
is the electrical efficiency of the machine? The eflaciency of 
conversion ? 

Ans. Electrical efficiency, 96 per cent. 

Efficiency of conversion, 93.8 — per cent. 

10. What is the commercial efficiency of a machine that 
will deliver 500 amperes at 550 volts when the input 'is 400 
horse-power? Ans. 92.1 + per cent. 



CHAPTER X 

DIRECT-CURRENT MOTORS 

200. Fundamental Principle of the Direct-Current Motor. — 
If a conductor in which there is a direct current be placed in 
a magnetic field in such a position that it makes an angle with 
the direction of the field, there will be a force tending to move 
the conductor. This same force is present in the case of a 
generator, but it is overcome by the mechanical force that 
drives the machine. With an increase in current in the con- 
ductor or an increase in the strength of the magnetic field, 
there will be an increase in the force which tends to move the 
conductor. 

201. Fleming's Left-Hand or Motor Rule. — There is a 
definite relation between the direction of current in a con- 
ductor, the direction of motion, and the direction of the mag- 
netic field for a motor; just as there is a definite relation 
between these three quantities in the case of a generator. If 
the thumb and first and second fingers of the left hand be 
placed at right angles to each other, the second finger pointing 
in the direction of the current in the conductor, the first finger 
in the direction of the magnetic field, then the thumb will 
point in the direction in which the conductor will tend to 
move. This simple rule is known as Fleming's left-hand or 
motor rule. If the direction of the current in the conductor 
be reversed, the direction of the magnetic field remaining 
constant, the direction of motion will be reversed; or, if the 
direction of the magnetic field be reversed, the direction of 
the current remaining the same, the direction of mo don will 
be reversed. If, however, the direction of the current and 
the direction of the magnetic field are both reversed, the 
direction of motion of the conductor will remain the same. 
Fig. 156 illustrates Fleming's left-hand rule. 

202. Generator and Motor Interchangeable. — The essential 
parts of a direct-current motor are identical with those of a 

193 




194 PEACTICAL APPLIED ELECTKICITY 

generator, namely, an armature and magnetic field. The con- 
nection of the armature conductors to the external circuit is 
made by means of a commutator which serves to reverse the 
direction of current in the armature 
winding at the proper time so that the 
forces tending to move the various con- 
ductors in the magnetic field all act to- 
gether and a continuous rotation of the 
armature is produced. Any direct-current 
Dir^ion^\! M^etic generator may be used as a direct-cur- 
of Motion^_^V^^_^^ jieid j,^^^ motor Or vice versa, their construc- 
Current tlou being practically the same. 

Fig. 156 203. Classes of Motors. — Direct-cur- 

rent motors may be divided into three 
main groups according to the method employed in exciting 
the field magnets. These are: 

(a) Shunt motors. 

(b) Series motors. 

(c) Compound motors. 

(a) The field windings of a shunt motor consist of a large 
number of turns of small wire connected directly across the 
terminals of the machine, or the line to which the machine is 
connected. The current in the field winding of a shunt 
machine is independent of the current in the armature so 
long as an Increase in armature current produces no change 
in the voltage impressed upon the shunt field winding. The 
field strength of the shunt machine is regulated by changing 
the current in the field winding, which may be done by either 
changing the impressed voltage or the total resistance of the 
circuit. 

(b) In the case of the series motor, the field winding con- 
sists of a few turns of large wire connected directly in series 
with the armature and the line. The current in the field 
windings is the same as the current in the armature, and the 
field strength varies with the load on the machine, the field 
current increasing with the load. 

(c) The field windings of a compound motor are a combi- 
nation of the shunt and the series windings. The magnetic 
effect of these two windings may aid or oppose each other, de- 
pending upon the way they are connected. When the two 
magnetizing effects act together, the machine is called a cumu- 



DIRECT-CUEEENT MOTOES 



195 



lative compound motor; and when their magnetizing effects 
oppose each other, the machine is called a differential com- 
pound motor. In the case of the cumulative compound 
machine, the field strength increases with an increase in load 
since the two magnetizing effects act together; and in the 
case of the differential compound motor, the field strength 
decreases with an increase in load since the two magnetizing 
effects act opposite to each other. 

204. Direction of Rotation of iVIachines when Clianged from 
a Generator to a iViotor. — The direction of current in the 
armature, field winding, and line for a self-excited shunt 
generator, and the polarity of the machine and direction 
+ — 




Fig. 157 




Fig. 158 



of rotation are shown in Fig. 157. The arrow (la) repre- 
sents the direction of the armature current. Let this ma- 
chine be operated as a motor by connecting it to some 
source of energy, connecting the positive terminal of the 
machine to the positive line, and the negative terminal of 
the machine to the negative line. The direction of current 
in the field and armature for this connection is shown in 
Fig. 158. It will be seen by inspection that the direction of 
current in the field winding has remained the same in the two 
cases and that the direction of current in the armature has 
changed. Now applying Fleming's dynamo rule to Fig. 157 
and his motor rule to Fig. 158 — remembering the direction of 
current in the armature in one case is just the reverse of 
what it is in the other, and the field current is the same in 
both cases — you will find the direction of rotation of the arma* 
ture in Fig. 158 will be the same as in Fig. 157. 

In the change from a generator to a motor, as shown in 
Figs. 157 and 158, the polarity of the terminals of the motor 
was the same as that of the generator. When the polarity of 



;96 PRACTICAL APPLIED ELECTEICITY 



the motor is just the reverse of that of the generator, the 
current in the shunt-field winding will be reversed in direc 
tion and the armature current will not change. This will also 
result in the direction of rotation of the armature remaining 
the same. Hence, a shunt generator will always run in the 
same direction when operated as a motor, as it did when it 
was run as a generator. 

To change the direction of rotation of a shunt generator 
when it is changed to a motor, the connections of the arma- 
ture or shunt-field winding must be reversed. 

When a series generator is changed to a series motor, the 
direction of rotation will be reversed, because the direction of 
the current in the field winding and the armature bear the 
same relation to each other in both cases and the motion will 
be opposite as shown by the right- and left-hand rules. Chang- 
ing the connection of the machine to the line will not change 
the direction of rotation as the current in both the armature 
and field winding is reversed when such a change is made. 
Then in order to change the direction of rotation of the 
armature, the connections of either the series field winding 
or armature must be reversed, which will result in a change 
in direction of either the tnagnetic field or of the armature 
current, but not of both. 

The compound generator will act, as far as direction of 
rotation is concerned, when changed to a motor, the same as 
though it were a simple shunt machine, provided the machine 
is lightly loaded. If it is started under a heavy load there 
will be an excessive current in the armature and series-field 
winding, and if the magnetizing effect of the series-field 
winding is greater than that of the shunt-field winding, the 
machine will start up as though it were a series motor. 

205. Armature Reaction in a Motor. — Let us assume th^t a 
shunt generator is operated as a shunt motor, the polarity of 
the machine being the same in both cases. The current in 
the shunt-field winding will remain constant in direction and, 
as a result, the direction of the magnetic field of the machine 
does not change. The direction of current in the armature, 
however, changes and as a result the direction of the magnetic 
field produced by it changes. When the brushes are in the 
normal neutral plane, as shown in Fig. 143, the field produced 
by the armature current is at right angles to that produced by 



I 

tie I 
jc- i 



DIEECT-CURRENT MOTORS 



197 




the field current and it is acting downward, as shown in Fig. 
159, instead of upward, as shown in Fig. 143. Since the mag- 
netizing effects of the armature current and the field current 
are present at the same 
time, they form a resultant 
field whose general direc- 
tion is similar to that 
shown in Fig. 160. It will 
be seen that the magnetic 
field in the case of a motor 
is shifted in a direction op- 
posite to the direction of 
rotation, which Is just the 
reverse of what occurred 
in the generator, as shown 
in Fig. 144. This results 
in the neutral plane of the 
magnetic field being shift- 
ed back of the normal 
neutral plane, as shown by the line (FG) in Fig. 160. 

206. Position of the Brushes on a Motor. — Since the neutral 
plane of the magnetic field is changed when the generator is 
changed to a motor, the plane in which the brushes are placed 

must be changed so that 
p it will correspond more 

nearly to the position of 
the neutral plane. The 
brushes then will be given 
an angle of lag in the case 
of a motor while they were 
given an angle of lead in 
the case of a generator. 

The demagnetizing turns 
on the armature are in the 
angle (29), as shown in 
Fig. 146, and the cross- 
magnetizing turns are those 
outside of this double an- 
gle. The turns in the dou- 
ble angle (26) are still demagnetizing, although the current 
through the armature has been reversed, for the following 




198 PEACTICAL APPLIED ELECTEICITY 



\ 



reason: When the machine is used as a generator the brushes 
are in advance of the neutral plane, as shown in Fig. 145, 
and when it is used as a motor they are back of the neutral 
plane, as shown in Fig. 160. If the brushes were changed from 
one position to the other without reversing the current in the 
armature, the current in the turns located in the angle (29) 
would be reversed in direction and the magnetizing effect of 
the ampere-turns in this angle (20) would act with the mag- 
netic field of the machine. The current in the armature, how- 
ever, is reversed at the same time the position of the brushes 
is changed and, as a result, the magnetizing effect of the 
turns in the angle (29) does not change in direction. 

The brushes are usually placed a little back of the neutral 
plane in the case of a motor for the same reason they are 
placed in advance of the neutral plane in the case of a 
generator, as explained in section (192). 

207. Torque Exerted on Armature. — The torque of a motor 
is equal to the product of the total force acting on the arma- 
ture conductors times the distance of the conductors from the 
center of the armature, or 

T = FXL (107) 

When the force (F) in the above equation is measured in 
pounds and the distance (L) between the point of application 
of this force and the center of the armature is measured in 
feet, the torque (T) will be given in pound-feet. The follow- 
ing equation can be used in calculating the torque in pound- 
feet in terms of the total number of conductors (Z) on the 
armature, the number of poles (p), the magnetic flux per pole 
($), the number of paths in parallel through the armature (b), 
and the total armature current (L). 

.1174 XpXZX*J>Xla 

T = (108) 

108 X b 

208. Mechanical Output of a Motor. — The output of a motor 
In foot-pounds per second is equal to the torque (T) in pound- 
feet multiplied by the speed in revolutions per second (r.p.s.) 
times 2 tt. Since one horse-power is equal to 550 foot-pounds 
per second, then the output of a motor in horse-power (h.p.) 
can be calculated by the use of the following equation: 



DIRECT-CUERENT MOTORS I99 

27r X T X (r.p.s.) 

h.p. = (109) 

550 

K the speed is measured in revolutions per minute (r.p.m.), 
then 

27r X T X (r.p.m.) 

h.p. = (110) 

33000 

209. Counter Electromotive Force. — When a machine is 
being operated as a motor, the armature is revolving in a 
magnetic field and there will be an induced e.m.f. set up in 
the conductors just the same as there would be if the machine 
were operated as a generator. Since the relation between 
the direction of motion of the conductors with respect to the 
direction of the magnetic field in the case of a motor is 
opposite to what it is in the case of a generator, the direction 
of the current in the conductors remaining constant, the 
induced e.m.f. in the armature of the motor will be just the 
reverse of what it is in the case of the generator. This e.m.f. 
opposes the flow of the electricity in the armature and hence 
takes energy from it, just as a force that acts in a direction 
opposite to the velocity of a body takes energy from the body, 
hence the motor action. This induced e.m.f. acts in a direc- 
tion just opposite to the impressed e.m.f. at the terminals 
of the machine and for that reason it is called a counter 
electromotive force. Its value depends upon the same factors 
as the e.m.f. of a generator, and it may be calculated by the 
use of equation (100). A counter e.m.f. is absolutely neces- 
sary to the operation of a motor. 

210. Normal Speed of a Motor. — The current in the arma- 
ture of a motor depends upon the resistance of the circuit, 
and the effective electromotive force acting in the circuit. If 
the impressed voltage on the machine be represented by (E), 
the counter electromotive force by (Ec), and the effective 
electromotive force by (Ef), then 

Ef = E — Ec (111) 

The current (la) in the armature is equal to 

Ia=-- (112) 

Ra 



200 PRACTICAL APPLIED ELECTRICITY 

or 

E -^— Ec 

la = (iisy 

Ra 

In the above equations (Ra) represents the total resistance 
between the terminals of the machine, neglecting the shunt 
field. In a shunt machine (Ra) would be the resistance of the 
armature, while in a series and compound machine (Ra) would 
be the resistance of the series field and armature combined. 
Since the current is dependent upon the counter electromotive 
force, as shown in equation (113), the machine will run at 
such a speed that the difference between the impressed voltage 
(E) and the counter electromotive force will produce suf- 
ficient current in the armature to produce the required torque 
in order that the machine may carry its load. Thus, with an 
increase in load on a machine there will be an increase in 
torque required, and this increase in torque will mean an 
increase in armature current if the field strength remains 
constant. Now in order that the current in the armature 
increase, the resistance (Ra) and impressed voltage (E) 
remaining constant, the value of the counter e.m.f. must 
decrease. The only factor in equation (100) that can change 
is the speed, since the field strength, or flux per pole (€>), is 
supposed to remain constant and the other factors are gov- 
erned by the construction of the machine and cannot be 
changed without rebuilding. There will then be a reduction 
in the speed of a machine with an increase in load current, 
all other factors remaining constant. 

211. Methods of Regulating the Speed of a Motor. — The 
speed of a motor may be regulated by any one, or certain 
combinations of the following methods: 

(a) Change in field strength produced by a change in field 

current. 

(b) Change in field strength produced by a change in the 

reluctance of the magnetic circuit. 

(c) Varying voltage over the armature by means of a 

rheostat. 

(d) Multi-voltage system. 

(e) By changing the position of the brushes. 

(a) A rheostat placed in series with the shunt field of a 



DIRECT-CURRENT MOTORS 



301 




R 

Shunt 
Field 



Fig. 161 



motor, as shown in Fig. 161, may be used to change its speed. 
If the resistance of the shunt-field circuit be increased by 
increasing the part of the resistance (R) in circuit, the field 
current will be decreased and there will be a decrease in the 
magnetic flux ($) per pole which 
will result in an increase in speed, 
all other quantities remaining con- 
stant, in order that the required 
counter e.m.f. may be generated in 
the armature. The change in the 
value of (^) due to a change in the 
field current will depend upon the 

degree to which the iron of the magnetic circuit of the ma- 
chine is saturated. If the circuit is well saturated there must 
be a relatively large change in field current to produce a 
small change in speed. 

There is a limit, however, to the amount you can weaken 
the field of a machine as the armature reaction increases with 
a decrease in field strength which results in serious sparking. 
The effect of armature reaction can be neutralized as ex- 
plained in section (191) and the allowable range in speed ob- 
tainable by this method thus greatly increased. A number 
of different kinds of field rheostats are described in the chap- 
ter on operation. 

(b) The magnetic fiux 
in the magnetic circuit of 
a machine can be changed 
by varying the reluctance 
of the magnetic circuit. 
This is accomplished in the 
case of a motor manufac- 
tured by the Stow Manu- 
facturing Company, in the 
following way: The field cores are hollow and are provided 
with movable iron cores. These cores are all connected 
mechanically so that their position in the field coils can be 
adjusted by means of a hand wheel on top of the machine. 
By moving them toward or away from the armature there will 
be a decrease or increase in the reluctance of the magnetic 
circuit and, as a result, an increase or decrease in the flux (*) 
per pole. This change in (*) will produce a change in the speed. 




Shunt 
Field 



Fig. 162 



202 PEACTICAL APPLIED ELECTRICITY 



• 



na.nP^^^"'^^ 



(c) If a rheostat (Ri) be placed in series with the armature 
of a motor, as shown in Fig. 162, the voltage across the 
terminals of the armature circuit can be varied by changing 
the resistance in the rheostat. A change in impressed voltage 
on the armature will mean a change in speed, because there 
will be a change in the value of the counter e.m.f. required. 
The Ward Leonard system, as described in section (213), is a 
form of variable voltage control. 

(d) In the multi-voltage method 

|v|g^j^y^ of speed control, there are several 

T" different voltages available from 

Main^ Z4oVolt5 which the motor may be operated. 

^•r ^olts Thus, as shown in Fig. 163, there is 

"^'"^ a different voltage between the dif- 

I ferent lines and these may be com- 

bined, giving other voltages, which 
Fig. 163 may be connected to the motor ter- 

minals by means of a suitable switch, 
or controller. This method is usually used in combination 
with the field rheostat method. 

(e) If the brushes of a machine be shifted from the neutral 
plane of the magnetic field, there will be an increase in the 
speed for the following reason: The counter e.m.f. between 
the brushes of a motor is a maximum when the brushes are in 
the neutral plane because the e.m.f. induced in ail the con- 
ductors, in series in the various paths through the armature 
windings, are acting in the same direction. If the position of 
the brushes be changed, it will result in the e.m.f. induced in 
some of the conductors in series opposing the e.m.f. in the 
others; and the resultant e.m.f. will be less than in the pre- 
vious case, all other conditions remaining constant. Now as 
the brushes are shifted from the neutral plane, the speed must 
increase in order that the counter e.m.f. between the brushes 
may satisfy equation (113). This is not a practical method for 
varying the speed, as excessive sparking usually results when 
the brushes are moved very much from their proper position. 
The brushes, in the case of a motor, can be placed in the 
neutral plane by moving them back and forth, noting the 
change in speed. The position giving a mininium speed will 
correspond to the neutral plane (no load on the motor). 



DIEECT-CURRENT MOTORS 



203 



212. Interpole Motor. — In order to prevent a shift in the 
position of the neutral plane of the magnetic field of a motor, 
due to a change in armature current, which tends to distort 
the field, commutating-poles, or interpoles, are used. These 
poles are placed between the regular poles of the machine, 
and the windings on them carry the load current. Their mag- 
netizing effect counteracts that of the armature current and 
the position of the brushes need not be changed with the 
change in load on the machine. If the direction of rotation of 
the armature be changed by changing the direction of the 
armature current, the polarity of the interpoles will also be 
changed and their magnetizing effect will still counteract that 
of the armature current. 

213. Ward Leonard System. — In this system the field of 
the motor (M) is connected directly to the main line and the 




Fig. 164 



armature is connected to an auxiliary generator (G), whose 
voltage can be regulated by means of the field rheostat (R), 
Fig. 164. In this arrangement the speed of the motor (M) is 
controlled by a change in impressed voltage, which in turn 
is controlled by the excitation of the generator (G). The 
field current for the generator (G) is taken directly from 
the line and its value is regulated by means of the rheo- 
stat (R). 

214. Comparison of Methods of Speed Control. — The field 
rheostat method is perhaps the cheapest and simplest method 
, of speed control and for that reason is no doubt used more 
than any of the others. It permits of a wide variation in 
speed when used with the interpole motor and the change 
in the speed can be made very gradually. 

The change in reluctance method is quite satisfactory, but 



PEACTICAL APPLIED ELECTEICITY 



the initial cost of the machine is usually prohibitive foH 
general use. ^' 

The armature rheostat method is very little used, as there 
is a large change in speed, with a change in load, and there 
is an excessive loss in the resistance for large armature cur- 
rents. 

The Ward Leonard and multi-voltage systems are very sat- 
isfactory in operation, but they are expensive to install. 

A change in speed produced by shifting the brushes i^^ 
not practical on account of difficulties due to sparking. HI 

215. Starting Motors. — There is no counter e.m.f. generated 
in the armature of a motor when it is stationary and if the 
machine were connected directly to line a very destructive 
current would exist in the armature. The value of this cur- 
rent, just at the instant the circuit was closed, would be 
equal to (E) divided by (Ra) if there were no resistance 
in series with the armature. The resistance of the arma- 
ture is usually very small and, as a result, the current would 
be large. By placing a resistance (Rx) in series with the 
armature, the current can be reduced to a safe value. Now 
as the armature starts to rotate there will be a counter 
e.m.f. generated and the effective e.m.f. acting in the circuit 
will be reduced, which will cause a reduction in current, and 
the speed will become constant in value when the effective 
e.m.f. acting in the circuit is equal to the product of the 
current and the total resistance, or 

E — Ee = la (Ra + Rx) (114) 

In the above equation (Ra) represents the resistance of the 
armature and (Rx) the resistance connected in series with 
the armature. If the resistance (Rx) be decreased, there 
must be an increase in speed, the armature current (la) re- 
maining practically constant, in order that the effective e.m.f. 
(E — Ec) will be equal to the (LR) drop in the armature 
circuit. When all of the resistance (Rx) is cut out of the 
circuit, the effective e.m.f. is equal to the armature current 
(la) times the armature resistance, or 

E — Ec = IaRa (115) 

and 

E — Ec 

Ia= (116) 

Ra 



DIRECT-CUERENT MOTORS 



305 



Supply 
Mains 




Fig. 165 



The field circuit of the motor must, of course, be closed when 
it is being started in order that the armature conductors 
may cut lines of force and have a counter e.m.f. induced 
in them, and also that the proper torque be generated to 
cause the armature to rotate. 

216. Starting Boxes, or 
Rheostats. — A resistance 
that can be connected in 
the circuit leading to the 
motor and so constructed 
that it may be slowly cut 
out as the motor speeds up, 
is called a starting box, or 
starting rheostat. A simple 
starting rheostat is shown 
in Fig. 165. The shunt field 
is connected directly to the 
supply mains and the arma- 
ture is connected through 
the resistance (Rx). When 
the switch (S) is first closed, all of the resistance (Rx) 
should be in circuit and as the motor speeds up, it may be 
gradually reduced and finally all cut out. 

217. Dead-Line Release. — In the operation of a shunt motor, 
as shown in Pig. 165, the armature may be destroyed by an 
excessive current resulting from the line becoming dead for 
a short time, which results in the speed of the motor decreas- 
ing immediately, and when the full line voltage is again ap- 
plied the counter e.m.f., on account of the decrease in speed, 
will not be of sufficient value to prevent an excessive cur- 
rent in the armature. To prevent the above condition oc- 
curring, the starting box can be provided with what is called 
a no-voltage or dead-line release magnet, as shown in Fig. 
166. The winding of this magnet may be connected in 
series with the shunt field winding, as shown in the figure; 
however, in adjustable speed motors, it is usually con- 
nected directly across the line. The arm (A) is moved 
from its initial position against the action of a coil spring 
and it is held in the extreme right-hand position by the 
magnet (M). If the current in the winding of (M) be reduced 
to such a value that the magnet will no longer hold the arm 



306 



PEACTIGAL APPLIED ELECTEICITY 







Q^^irJ.(^^-l^^-p 





(A), the arm will be released and it will return to its initial 
position. This will result in the armature and field circuits 
both being opened if the line to which the motor is connected 
should become dead. The arm (A) must then be moved by 
an attendant to its right-hand position in order to operate the 
motor when the line becomes alive. 

218. Overload Release. — 
The armature of a motor 
may be burned out, due to 
an excessive current pro- 
duced by the motor being 
overloaded, and the purpose 
of the overload release is to 
open the circuit or discon- 
nect the motor from the 
line before the machine is. 
injured, due to an excessive 
current. The overload re- 
lease magnet winding car- 
ries ttre armature current 
and when this becomes ex- 
cessive a piece of iron is 
attracted, which shorts the 
no-voltage release magnet 
and allows the arm to re- 
turn to its initial position. 
A starting box equipped with an overload release is shown 
in Fig. 167. 

219. Combined Starting and Field-Regulating Rheostats. — 
In a rheostat of this kind a * field-regulating resistance is 
combined with a starting box. Such a rheostat as manufac- 
tured by the Cutler-Hammer Manufacturing Company is 
shown in Fig. 168. The movable arm consists of two parts 
and their outer ends move over separate sets of contacts. 
When the motor is being started, the arm is moved to the 
extreme right-hand position and the lower portion is held 
there by the no-voltage release magnet, while the upper por- 
tion may then be moved back over the upper row of contacts 
which are connected to the field-regulating resistance. A 
diagram of this starting, box is shown in Fig. 169. 

220. Speed Regulation. — The speed regulation of a motor 



Fig. 166 



DIRECT-CUREENT MOTORS 



207' 



is the change in speed from full load to no load expressed 
as a percentage of the full load speed, the field resistance 
and impressed voltage remaining constant. Thus, if the 
speed of a shunt motor is 1000 r.p.m. at full load and 1050 
r.p.m. at no load, its speed regulation is 
1050 — 1000 

X 100 = 5 per cent 

1000 
In order that the speed of the machine may remain constant 
with a change in load the field strength of the machine must 
be changed. 

Characteristics of the Shunt 
Motor. — The change in torque, 
current, and speed that takes 
place as the load on a shunt 
motor changes is shown in Fig. 
170. There is a decrease in 
speed with an increase in load. 
The current and torque both in- 
crease at about the same rate, 
with an increase in load, since 
the field strength remains prac- 
tically constant. 

221. Characteristics of the Se- 
ries Motor. — The speed of a 
series motor will drop off a 
great deal more with an increase 

in load than in the case of a shunt motor because there is 
an increase in field strength with an increase in load. Care 
should be taken in the operation of series motors to see that 
their load never drops to zero while they are connected to 
the source of energy, as their speed will become excessive 
or they will "race." If the iron of the magnetic circuit is 
worked near saturation or above the "knee" of the mag- 
netization curve, the cTiange in speed will not be as great 
as it is when the iron is not so near saturation. The in- 
crease in torque with an increase in load must necessarily 
be greater than in the case of a shunt motor, as the decrease 
in speed is greater. The torque is proportional to the arma- 
ture current and field strength, and the field strength varies 
almost directly a? the armature current, the series winding 




Fig. 167 



208 



PBACTICAL APPLIED ELECTKICITY 



be-S' 



and armature being connected in series. If the iron is 
low the knee of the magnetization curve the torque will 
then vary practically as the square of the current; when, 
however, the magnetic circuit is worked above the knee of 
the curve, the variation in torque due to a change in current 
becomes less. The change in torque, current, and speed that 
takes place as the load on a series motor changes, is shown 

in Fig. 171. 

222. Characteristics 

of the Compound iVIotor. 
When the compound 
motor is connected so 
its two fields aid each 
other, its characteristics 
are between those of 
the shunt and the series 
motor. If the magnet- 
izing action of the se- 
ries field opposes that 
of the shunt field, the 
speed remains nearer 
constant with an in- 
crease in load than it 
does for a shunt motor. 
The torque does not 
need to increase so rapidly as a result of the speed remaining 
constant, as it does in the case of a shunt motor. The arma- 
ture current must increase, since the fieid strength is de- 
creased in order that the same torque be produced in the ease 
of a shunt and differential compound motor. 

223. Adaptability of Different Motors. — There are three 
different classes of work to be performed by motors, each 
requiring a different relation between motor torque, and 
speed. Examples of these classes are as followc: 

(a) When a motor is used on a crane or an elevator it 
must be capable of developing a constant torque at a variable 
speed, since it is desired to move a given weight at different 
speeds. 

(b) In some classes of work, such as the operation of a 
motor used in driving generators at a constant speed, the 
motor will be subjected to a varying load as the output of 




Fig, 168 



DIEECT-CUEEENT MOTOES 



209 



the generator is changed, but its speed is to remain constant 
and hence it must be capable of developing a variable torque 
in order that it may carry the load. 

(c) In certain classes of work the motor will be required 
to develop a variable torque at a variable speed. Such is 
the case in street-car motors. The torque required is a 
maximum when the car is being started and the speed a 
minimum, and as the speed increases the torque decreases. 

Motors may then be divided into three classes, according 
to the character of the work to be performed, and they must 
develop either: 



Constant 
at variable 



(a) 
torque 
speed. 

(b) 
torque 
speed. 

(c) 
torque 
speed. 

The shunt and the 
compound motor 
meet the require- 
ments in cases (a) 
and (b), and the 



Variable 
at constant 

Variable 
at variable 




Fig. 169 



series motor operated from a constant-potential line generally 
fulfills the requirements for case (c). The only motor that 
naturally meets the requirements in case (a) is the series 
motor operated on a constant-current circuit. 

224. Construction of Motors. — The character of the work 
a motor is to perform will determine to a great extent its 
construction. The motor may be located in an exposed place; 
or it may be in a room filled with steam or dust and, as a 
result, it must be practically enclosed in order to provide 
ample protection to the windings, etc. If a motor is to be 
used in a street car, for example, its mechanical construction 
would need to be quite different from one that was to be 
used on an elevator, as the street-car motor would, no doubt, 
be subjected to more mechanical abuse than the elevator 
motor. 



310 



PEACTICAL APPLIED ELECTEICITY 



225. Railway Motors and Their Control. — There are two 
general methods for controlling railway motors, viz, 

(a) The rheostatic method. M 

(b) The series-parallel method (in combination witM 
rheostat). 

(a) The connection in the case of the rheostatic method 
is shown diagrammatically in Fig. 172. Resistance is placed 

in series with the various 
motors and by cutting this 
resistance in and out of cir- 
cuit, the voltage impressed 
upon the motor can be 
changed. 

(b) The series-parallel 
method consists first in 
placing two motors in series 
with a resistance in circuit 
with them, as shown in Fig. 
173, then decreasing the re- 
sistance until the motors 
are connected in series di- 
rectly across the line. This constitutes what is called a 
running position as there is no (I2R) loss in a starting resist- 
ance, the entire voltage being impressed upon the two motors. 











c ^ 


■i _ On^N^v ^^ ~\ 


::::=:;=»-.r^G:: ::::;^::: 


;2===-2_ 


:"■ ■ : ~^^^ ^2V.'. : 


% _^^__^^! 


__ .r.-<S^^^ ,t^ - 


J<^^ ^^' ij 


_ csSsL^^ -^^' 


^lTT " ^ 


II- > ^r-Ta^^i I :i - ^ 


---^^- ^-^^O-Sl 


-^''--x'' -\ 


^ ^ 


'" -L 1 r\n^-i. 


1 hP^^ 



Fig. 170 



The next connection, places 
resistance in series with 
them, as shown in Fig. 174. 
This resistance is then grad- 
ually cut out and ,the mo- 
tors are finally operating 
directly across the line, 
which corresponds to the 
final running position. 

The control of the switches 
governing the connections 
may be accomplished di- 
rectly by hand or by an 
auxiliary control. In the 
first case the changes in 



the motors in parallel and a 




Fig. 171 



connections are made by a motorman on the car platform, who 
moves the handle of a controller. This movement of the con- 



DIRECT-CUERENT MOTORS 



211 



troller handle causes a cylinder inside the containing case to 
rotate. This cylinder has a number of contact arms mounted 
on its surface and these make contact with stationary fingers 
as the cylinder is rotated. A controller manufactured by the 
General Electric Company is shown in Fig. 175. A small handle 
to the right of the main controller 
handle, as shown in the figure, ena- 
bles the motorman to reverse the di- 
rection of rotation of the motors by 
changing the connections. The com- 
plete wiring diagram of a street car 
is shown in Fig. 176. 

When auxiliary devices are used 
to control the connections of the various motors the system is 
called a multiple-unit control. When this system is used, a 
uumher of cars can be coupled together and the motors on all 



Supply 



Fig-. 172 




Fig. 173 

of them controlled from any car. The equipment of each car 
consists of a series-parallel controller, whose electrical opera- 
tion is similar to that just described, and so arranged that it is 
controlled from master controllers that are located in the 
motorman's cabs at the ends of the cars. The leads that 
run from the m.otor controllers to the master controllers run 




Fig. 174 

the entire length of the train, the connection between cars 
being made by means of suitable couplers. The operation 
of any one of the master controllers will cause all of the 
motor controllers to operate in the same way and thus all 



212 



PBACTICAL APPLIED ELECTKICITY 



of the various motor connections throughout the train are 
the same. 

In the Sprague-General Electric Multiple-Unit Control Sys- 
tem, the master-controller circuit takes current direct from 
the line and the motor controllers are operated by means 
of solenoids. 

In the Westinghouse 
Electric Company's Multi- 
ple-Unit Control System, 
the motor controllers are 
operated by compressed 
air, the valves controlling 
the air being governed by 
the master controllers. 
The master-controller cir- 
cuit receives its current 
from a storage battery. 

226. Automobile Motors. 
— The series motor is uni- 
versally used for automo- 
bile work. The source of 
energy is a storage battery 
and the cells are so ar- 
^^^- ^'^^ ranged that they can be 

connected in different ways, giving different voltages. This 
variation in voltage, together with a series-parallel combina- 
tion of different sections in the series field affords an easy 
means of controlling the speed. Thus, if forty cells be used 
they may be connected in four groups of ten cells each 
and these groups then connected in parallel; as the speed 
of the motor increases, the grouping of the cells can be 
changed and also the series field connections. These ch'anges 
in connections are governed by a suitable controller so ar- 
ranged that the driver of the machine can operate it by a 
lever or hand wheel. 

Automobile motors are very compact and usually con- 
structed with a view to economy in weight. When they 
are located under the car, they are of the enclosed type; 
while if they are placed in an unexposed position, they can be 
of the semi-enclosed or open type. In some cases two mo- 
tors are provided, one being attached to each wheel through 







DIEECT-CURRENT MOTORS 



213 




Fig. 176 



214 



PRACTICAL APPLIED ELECTEICITY 



a suitable transmission, the wheels turning independent Of 
each other, while in other cases a single motor is employed, 
the connection to the wheels being made by means of some 
form of differential gear. The various parts of an automo- 
bile motor used by the Woods Electric Company are shown 
in Fig. 177. 




Fig. 177 



227. Elevator and Crane Motors. — There are many dif- 
ferent forms of elevator and crane motors on the market and 
the electrical operation of all of them is practically the sanie. 
In the majority of cases some kind of a brake is provided 
so that the load may be held after it is raised. These brakes 
may be either of the friction or of the dynamic type. 




Fig. 178 

The friction brake is usually controlled by a solenoid. When 
the motor is operating, the solenoid is energized and the 
brake does not act. If the motor is disconnected from the 



DIRECT-CURKENT MOTORS 215 

line and the solenoid de-energized, the brake clamps a pulley 
and prevents the armature turning. 

In the dynamic brake the motor is converted into a gen- 
erator and delivers current to some local circuit while it is 
coming to rest, which results in the motor stopping quicker 
than it would otherwise. The dynamic brake is usually used in 
combination with the friction brake, the friction brake be- 
coming operative after the action of the dynamic brake 
ceases. A motor provided with a friction brake is shown in 
Fig. 178. 

228. Efficiency of a Motor. — There are three efficiencies 
for a motor, namely, 

(a) Efficiency of conversion. 

(b) MecFianical efficiency. 

(c) Commercial efficiency. 

(a) The efficiency of conversion is the ratio of the total 
mechanical power developed to the total electrical power 
supplied (EI), or 

(P -f stray-power losses) 

, Efficiency of conversion ^ X 100 

EI 

(117) 
(P in the above equation is the power available at the pulley.) 

(b) The mechanical efficiency is the ratio of the mechan- 
ical power available at the pulley (P) to the total mechan- 
ical power developed, or 

P 

Mechanical efficiency = X 100 

(P + stray-power losses) 

(118) 

(c) The commercial efficiency is the ratio of the avail- 
able output (P) to the input (EI), or 

P 

Commercial efficiency = X 100 (119) 

EI 

The commercial efficiency is by far the most important 
of the three; it includes all the losses in the machine. 

229. Determining the Commercial Efficiency by Test. — The 
commercial efficiency of a motor can be determined by meas- 



216 



PEACTICAL APPLIED ELECTRICITY 



uring the electrical input by means of a voltmeter and an 
ammeter, and at the same time measuring the mechanical 
output. When the values of these two quantities are known, 
they may be substituted in equation (119) and the efficiency 
calculated. 

A satisfactory way of measuring the output of the motor 
is by means of a Prony brake. The construction and opera- 
tion of this brake can best be explained by reference to 
Fig. 179. The brake proper consists of two parts (C) and 
(D) that are held together by two bolts (Bi) and (B2). The 

bolts are provided with small 
hand wheels (Hi) and (H2) 
by means of which the pres- 
sure of the brake upon the 
pulley can be varied. An 
arm (A) is attached to the 
brake and extends out at 
right angles to the shaft 
upon which the pulley (P) is 
mounted. When the brake is 
in use, the outer end of the 
arm (A) rests upon the platform of a pair of scales. The 
torque exerted by the armature in pound-feet is equal to the 
net reading of the scales in pounds multiplied by the hori- 
zontal distance (L), in feet, between the point where the arm 
(A) rests upon the scales and the center of the shaft. Call 
the scale reading (W), then 

T = W X L 
and the output in horse-power will be equal to 

27r X T X r.p.m. 

h.p.= 

33 000 




Fig. 179 



(120) 



(121) 



or 



h.p. 



27r X W X L X r.p.m. 



33 000 



(122) 



The input, of course, will be in electrical units and either 
the output or input must be changed before equation (119) 
can be used in calculating the efficiency. 



DIRECT-CUEKENT MOTORS 217 

[Note. — The dead weight of the brake must always be sub- 
tracted from the scale reading in order to obtain the net 
weight or the value to be used in equation (122) ]. 

Example. — In the test of a motor by the Prony brake 
method, the net scale reading was thirty pounds, the lever 
arm of the brake was two feet, and the motor was running at 
1000 r.p.m. What was its commercial efficiency if the input 
was 91.0 amperes at 110 volts? 

Solution. — The output can be obtained by substituting in 
equation (122), which gives 

27r X 30 X 2 X 1000 

h.p. = 

33 000 
376 992 

= = 11.42 

33 000 
The output was then 11.42 horse-power or 8519.32 watts. The 
input was 

91 X 110 = 10 010 watts 
and the efficiency was 

8519.32 

= 85.1 

10 010.00 

Ans. 85.1 per cent. 

PROBLEMS ON DIRECT-CURRENT MOTORS 

1. Calculate the torque in pound-feet, exerted by the 
armature of a motor wound with 300 inductors (simplex lap 
winding, singly re-entrant), and revolving in a four-pole 
magnetic field, the flux (<l>) per pole being 4 000 000 maxwells 
and the armature current 200 amperes. 

Ans. 281.76 pound-feet. 

2. If the speed of the armature in problem (1) is 1200 
revolutions per minute, what is the horse-power output of 
the motor? Ans. 64.3 horse-power. 

3. Calculate the counter electromotive force generated for 
the data given in problem (1) if the speed is 1200 r.p.m. 

Ans. 240 volts. 



218 PRACTICAL APPLIED ELECTRICITY 

4. Calculate the Impressed voltage for the above motor, 
if the armature resistance is .035 ohms, (L) being 200 am- 
peres and (Ec) 240 volts. Ans. 247 volts. 

5. The speed of a motor is 1200 r.p.m. when operating 
under load and 1250 r.p.m. when operating without load, 
what is its speed regulation in per cent? 

Ans. 4.17 — per cent. 

6. Calculate the output of a motor in horse-power when the 
input is 100 amperes at 110 volts and the commercial effi- 
ciency of the machine is 90 per cent. 

Ans. 13.27 horse-power. 

7. At what speed must a motor run in order that its out- 
put be 50 horse-power, the torque being 263.0 pound-feet? 

Ans. 1000 revolutions per niinute. 

8. What is the average pull on each of the inductors in 
problem (1) if the inductors are located 8 inches from the 
center of the armature? Ans. 1.41 — pounds. 

9. The total armature resistance of a six-pole 250-volt 
direct-current motor, including brushes, brush contacts, etc., 
is .012 ohm. The winding on the armature is a simplex 
singly re-entrant lap winding and composed of 600 inductors. 
What is the flux (<I>) per pole when the machine runs at 900 
r.p.m. and takes a current of 300 amperes? 

Ans. 2 738 000 maxwells. 

10. In testing a motor by the Prony brake method, the 
following data were obtained. Calculate the commercial effi- 
ciency for this particular load: 

(Net) Scale reading (W) = 38.88 pounds 

Length lever arm (L) =3 feet 

Speed of armature, 900 revolutions per minute 

Impressed voltage, 220 volts 

Current input, 75 amperes Ans. 90+ per cent 



CHAPTEE XI 

ARMATURES FOR DIRECT-CURRENT DYNAMOS 

230. Armature. — An electrical conductor, such as a sim- 
ple loop of wire, moved in a magnetic field, so that there is 
an e.m.f. induced in it, constitutes a simple armature. An 
armature composed of a single turn of wire is shown in Fig. 
127. The e.m.f. induced in such an armature is unsteady — see 
section (178) — and in order that a practically steady e.m.f. 
be induced more turns of wire should be used and these vari- 
ous turns should be so located and connected with respect to 
each other that the e.m.f. between the brushes remains almost 
constant. 

The armature of a commercial direct-current dynamo con- 
sists of three principal parts, namely, 

(a) The armature core (sections 231 to 233 inclusive), 

(b) The commutator (sections 234 to 236 inclusive). 

(c) The armature windings (sections 238 to 251 inclusive). 

231. Armature Cores. — The core of an armature serves a 
double purpose: it supports the armature winding and it con- 
ducts the magnetic flux from the face of one pole piece to 
another. 

Since the armature core is moved relative to the magnetic 
field, it becomes magnetized in alternate directions, which 
results in a loss due to hysteresis. There will also be a loss, 
as explained in section (127), due to eddy currents. The 
hysteresis loss can be reduced to a minimum by using a 
grade of iron whose hysteretic constant is low, while the 
eddy-current loss is reduced by building the armature cores 
of thin soft iron or sheet steel disks insulated from each 
other by rust, insulating varnish, or paper. The disks are 
always placed in such a position with respect to the magnetic 
field that their plane is parallel to the magnetic fiux. 

Armature cores are of two general types, smooth cores and 
slotted, or tunneled, cores. In the smooth core type, the arma- 

219 



220 



PEACTICAL APPLIED ELECTEICITY 



ture windings are placed upon the surface of the cores, 
while in the slotted, or tunneled, type, they are placed in slots 
or openings cut in the outer edge of the core stampings. 
The mechanical and electrical construction of the slotted^ 
or tunneled, type is better than that of the smooth core 





Fie. 180 



Fig. 181 



type, because the length of the air gap in the magnetic circuit 
is reduced, and the conductors are held rigidly in place, 
which prevents their moving back and forth and the likeli- 
hood of injury to the insulation on them is thus greatly 
reduced. 

232. Armature-Core Stampings. — The stampings used in the 
construction of armature cores assume a number of differ- 





Fig. 182 



Fig. 183 



ent forms, depending upon the size and kind of armature 
for which they are intended. In the construction of small 
armature cores the disks are punched in one piece, as shown 
in Figs. 180 and 181. The disks used in the construction 
of large cores are made in sections, as shown in Figs. 182 
and 183, and these various sections are then mounted upon 



ARMATURES 



321 



an auxiliary support called a spider, which may assume a 
number of different forms. A spider upon which the disks 





Fig. 184 



Fig. 185 



shown in Fig. 182 can be mounted is shown in Fig. 184. 
Such a core would usually be used for a ring winding. A 
spider upon which the disks shown 
in Fig. 183 can be mounted is 
shown in Fig. 185. The dovetail 
notches or extensions on the stamp- 
ings fit into dovetail extensions or 
notches on the spider arms. The 
various lamina? are held together 
by means of bolts (B) and end 
plates (Pi) and (P2), as shown in 
Fig. 188. 

233. Ventilation. — On account of 
the heat generated in the armature 
cores, due to hysteresis and eddy 
currents in the iron, and the (I2R) 
losses in the armature conductors, 
some means of ventilation must be 
provided in order to prevent an ex- 
cessive temperature rise. The 
means usually employed in venti- 
lating the armajture cores, especially in the larger machines 
is to separate the cone disks at certain intervals along 
the axis of the core. These openings between the disks 




Fig. 186 



222 



PKACTICAL APPLIED ELECTEICITY 



called ventilating ducts, can be made by placing a piece of 
metal on edge, as shown in Figs. 187 and 188, and fastening 
them to the disk. These ventilating ducts are usually spaced 
from 2 to 4 inches apart. 





it 



Fig. 187 



Fig. 188 



234. The Commutator. — The commutator usually consists 
of a number of similar wedge-shaped pieces of copper clamped 
between two rings, as shown in Figs. 189 and 190, and in- 
sulated from each other by mica or other insulating material 
whose wearing qualities are practically the same as that of 
copper. Amber mica is usually used for this purpose. 





Fig. 189 



Fig. 190 



The commutator shown in Fig. 189 is for a small machine, 
while the one shown in Fig. 190 is for a large machine. 

235. Commutator Risers. — The various commutator seg- 
ments are connected to the armature winding by means of 
commutator risers. These risers consist of pieces of metal 
that project radially from the end of the commutator toward 
the surface of the armature. In the case of small machines 
they are formed as a part of the commutator segment. In 
large machines a piece of metal is soldered or brazed into a 



J 



AEMATURES 223 

groove cut in the end of the commutator bar before the com- 
mutator is assembled. 

236. Construction of the Commutator. — In constructing the 
commutator the segments and insulation are placed inside 
a heavy clamping ring and the inside of the commutator is 
turned down to the proper dimension. It is then clamped 
between its own rings (Rj) and (R2), Fig. 189, before the 
first clamping ring is removed. When it is rigidly fastened 
in place, the outer ring may be removed and the outer 
surface turned down to the proper dimensions and form. 
^ 237. Brushes and Brush Holders. — The brushes used on 
dynamos are usually made from hard blocks of what is known 
as graphitic carbon. In some special cases, however, metal 
brushes are used, especially when a very low resistance 
brush is desired, or they may be a combination of metal and 
carbon. The carbon brushes have the advantage of wearing 
well mechanically and they are self-lubricating, giving the 
commutator a very smooth surface. They also have a higher 
resistance than the metal, or combination brushes and, as 
a result, reduce the tendency for sparking to occur at the 
commutator when the brush bridges two commutator seg- 
ments connected to an element of the armature winding that 
may be undergoing commutation. The tendency for spark- 
ing at the commutator, due to the cause just mentioned, is 
not very great in low voltage machines, such as those used 
in electroplating and, as a result, copper or copper gauze 
brushes are used, thus reducing the total resistance of the 
circuit. 

Brushes are usually mounted so that they make an angle 
with the surface of the commutator, although in some cases 
they are set radial, especially in cases where the direction 
of rotacioD of the machine is to change, as in street-car 
motors. 

The brushes are supported in individual holders, and these 
holders are mounted on arms called brush-holder arms, which 
in turn are mounted on rings that are concentric with the 
commutator and called rockers. The brush holders should 
be so constructed that the pressure of the brush on the 
commutator can be adjusted and the electricity be conducted 
to the brush holders through a path other than that formed by 
the spring used in holding the brush unon the commutator. 



224 



PBACTICAL APPLIED ELECTEICITY 





Fig. 191 



The rockers supporting the brush-holder arms are mounted 

on the front bearing in small machines, and on projecting 

arms from the magnetic frame 
in large machines. The rockers 
are always so arranged that they 
can be moved by a lever or a 
specially arranged hand wheel, 
which affords a means of ad- 
justing them to their proper po- 
sition on the commutator. A 
brush and brush holder are 
shown in Fig. 191. The brushes 
are connected to the stationary 
part of the holder by means of 

flexible copper conductors. 

238. Armature Windings. — ^Armature windings may be 

classified according to the manner in which they are placed 

upon the core, as follows: 

(a) Ring windings. 

(b) Drum windings. 

(c) Disk windings. 

(a) In ring windings 
the conductors are placed 
upon a ring-shaped core, 
the winding passing 
through the interior of the 
ring, as shown in Fig. 192. 
These windings are some- 
times referred to as hel- 
ical windings. 

(b) In drum windings 
the winding is placed en- 
tirely upon the surface of 
the drum, when smooth 

cores are used, or in slots or tunnels cut in the surface of the 
core. Such an armature winding is shown in Fig. 193. 

(c) In the disk windings the cores upon which the wind- 
ing is placed are shorter and larger in diameter in propor- 
tion than they are in the case of drum windings. The in- 
ductors correspond to the spokes of a wheel and are con- 




Fig. 192 



^ 



ARMATURES 



225 



nected in a manner similar to those in the drum winding. , A 
disk-wound armature is shown in Fig. 194. 
In addition to the above classification of armature wind- 




Fig, 



ings, they may be grouped into two types, depending upon 
whether the winding constitutes an open or closed circuit, viz. 




Fig, 194 



(a) Closed-coil windings. 

(b) Open-coil windings. 

(a) The closed-coil windings have all the inductors in- 



226 PRACTICAL APPLIED ELECTRICITY 

terconnected, so that the e.m.f. s induced in them are always 
effective in producing a current in the external circuit, ex 
cept when a certain part of the winding is undergoing com- 
mutation. This type of winding gives a very steady current 
and the difficulties due to sparking are considerably less 
than in the open-coil type. 

(b) The open-coil windings have the inductors and com- 
mutator segments so arranged that the e.m.f.'s induced in 
the windings are only effective in producing a current in 
the external circuit when the coil is undergoing commuta- 
tion. This type of winding does not give as steady a cur- 
rent as the closed-coil type and considerable trouble is en- 
countered in commutation due to sparking. The open-coil 
winding is used mostly (in the case of direct-current ma- 
chines) for constant-current arc-lighting machines. 

239. Armature Inductors. — That part of the armature wind- 
ing in which an e.m.f. is induced is called an armature in- 
ductor. The terms conductor and inductor have been used 
in the same sense up to the present time, but in speaking 
of armature windings from now on the term armature in- 
ductor will be used. Thus, in the case of a ring winding, 
as shown in Fig. 192, there will be one inductor per turn, it 
being the outer portion of the turn, as that is the only part 
of the turn in which there is an e.m.f. induced. In the drum 
winding, as shown in Fig. 193, there will be two inductors 
per turn, because both sides of the turn have e.m.f.'s in- 
duced in them. 

The e.m.f.'s induced in the conductors in the case of the 
ring winding are all acting in series and in the same direc- 
tion, the inductors being connected by the part of each turn 
that passes through the ring. If this return portion of each 
turn were placed on the outside of the cores, as in the case 
of a drum winding, it would become an inductor and there 
would be an e.m.f. induced in it, but the direction of the 
induced e.m.f. would be such, if the two parts of the coil 
w^ere under poles of the sam<^ polarity, that it would tend to 
neutralize the e.m.f. in the other portion of the coil or in- 
ductor. In order that the e.m.f. in the two parts of an arma- 
ture coil, in the case of a drum winding, may act in series 
and in the same direction, they are so placed on the core 
that the two parts are under poles of opposite polarity. 



AEMATURES 227 

240. Element of Armature Winding. — The element of an 
armature winding is that part of the winding which termi- 
nates at two consecutive commutator segments, as the wind- 
ing is being traced out. In a ring winding there may be a 
commutator segment connected between all the turns, then 
an element would consist of a single turn and one inductor. 
The number of inductors and turns per element in the case 
of a ring winding is the same, and the number of conductors 
or turns per element will depend upon the relation of the 
number of commutator segments (K) and the number of 
inductors (Z). In the drum winding a commutator segment 
may be connected between all the turns and an element 
would consist of a single turn, or two inductors. There 
will always be twice as many inductors per element as there 
are turns in the case of drum windings. 

241. Armature Coil. — In winding armatures a number of 
turns are usually placed together and insulated as a unit 
and this unit, called a coil, is placed upon the armature core. 
The method of forming and insulating the coils depends en- 
tirely upon the kind of armature they are intended for, their 
insulation being better and greater care is usually taken in 
their construction when they are to be used on a high volt- 
age machine. In some cases the winding is placed directly 
on the armature core. 

A coil may correspond to an element of the winding or 
the number of inductors per coil may be greater or less 
than the number of inductors per element. Thus, two or 
more coils may be connected in series to form an element, 
or taps may be taken off the coil, only part of the coil being 
used for an element. 

242. Number of Commutator Segments. — In the case of a 
simple ring winding, the maximum number of commutator 
bars (K) that it is possible to have is the same as the num- 
ber of turns or inductors on the armature cores. The num- 
ber of bars may, of course, be less than the number of in- 
ductors, as each element may be composed of a number of 
Inductors. In general, if (M) represents the number of 
turns in each element of the winding and (Z) the total 
number of inductors, the number of commutator segments 
will be equal to 



228 PEACTICAL APPLIED ELECTRICITY 

Z 

K = — (123) 

M 
The number (M) can have any value so long as (Z) divided 
by (M) gives a w^hole number as a quotient. 

The maximum number of commutator bars that it is pos- 
sible to have in a commutator to be used with a simple drum 
winding is equal to 

Z 

K = (124) 

2M 
In the above equation, (M) represents the number of turns 
in series in each element of the winding and since there are 
two inductors per turn for a drum winding, (2M) is the 
total number of inductors per element. The number (M) 
can have any value, even or odd, but (2M) will always be 
even and, since (K -=- 2M) must give a whole number, as in 
the previous case, (Z) must be even. 

243. Pitch of Winding and Field Step. — It was explained 
in section (239) that the two inductors composing a single 
turn in the case of a simple drum winding had to be placed 
in such a position on the armature that the inductors were 
under poles of opposite polarity. In the case of a ring wind- 
ing, as explained in section (239), the inductors can be placed 
under a pole of the same polarity because they are con- 
nected by a conductor that passes through the center of the 
ring and in which there is practically no induced e.m.f. The 
field step in the case of a winding is unity when the distance 
between one inductor and the n^xt in order, in tracing 
through the winding, corresponds very nearly to the distance 
between the centers of adjacent unlike poles. The field step 
then for a simple ring winding is zero and for a simple drum 
winding is unity. It may be greater than these values for the 
two windings, but the connections required to join the ends 
of the inductors would be unnecessarily increased in length, 
which would mean an increase in the cost of copper required 
for the winding and an increase in the resistance of the 
armature. 

The pitch of a winding is the distance from one inductor 
of the winding to the next inductor in order. This distance 
is usually measured in terms of the number of inductors 
passed over, or it may be measured in terms of the number 



ARMATURES 229 

of half coils, slots, or distance on the surface of the arma- 
ture. An example would perhaps show more clearly the 
exact meaning of the term "pitch." Let the inductors on a 
certain drum-wound armature be numbered consecutively 
around the armature starting with number (1). If inductor 
number (1) is joined at the back end of the armature (end 
opposite the commutator) to inductor (14), and inductor (14) 
Is joined at the front end of the armature to inductor (27), 
etc., the pitch of the winding would be the same at both ends 
and equal to 13. The front pitch is represented by the 
symbol (Yf) and the back pitch by symbol (Yb). These 
pitches are both positive in sign when they are measured 
in the same direction around the armature. If, in the above 
case, inductor (1) be joined to inductor (16) at the back 
end, and inductor (16) be joined to inductor (3) at the front 
end, and inductor (3) be joined to inductor (18) at the back 
end, etc., the front pitch would be negative and the back pitch 
positive. Their values would be (Yb) = 15 and (Yf) = — 13. 

The average pitch (Yav) in any case is equal to the aver- 
age of the front and the back pitches regardless of their signs. 
For the first winding (Yav) would be [(13+ 13) -^ 2] = 13, 
and for the second winding (Yav) would be [(15 + 13) -^2] 
= 14. The resultant pitch (Yr) of any winding is the alge- 
braic sum of the two pitches. For the first winding (Yr) 
would be (15) + (—13) = 2. 

The commutator pitch is the interval between the com- 
mutator segments connected to an element of the winding 
expressed in termg of the number of commutator segments. 

244. Types of Ring Windings. — There are two types of 
ring windings: 

(a) Spirally-wound ring windings. 

(b) Series-connected wave-wound ring windings. 

(a) In the spirally-wound ring armature, the winding forms 
a closed helix, as shown in Fig. 192. There will be (p) points 

on the commutator (p) represents the number of magnet 

poles where brushes may be connected to conduct the elec- 
tricity to and from the armature winding. The proper loca- 
tion of the brushes is shown in the figure and the number 
of paths (b) through such a winding is equal to the number 
of poles (p). The current capacity of this winding will be 
greater than that of a bipolar machine if the same size wire 
is used in the winding; and the e.m.f.'s will be the same if 



230 



PRACTICAL APPLIED ELECTKICITY 




Fig-. 195 



the field strength, speed, and number of inductors in series 
are the same in both cases. The e.m.f. equation for such a 
machine is 

Z X ^ X P X r.p.m. 

E = - (125) 

108 X 60 X b 

The number of brushes can 
be made less than the num- 
ber of poles by permanently 
connecting the various com- 
mutator segments together 
that are always at the same 
potential. 

(b) In the series - con- 
nected ring winding, the va- 
rious turns are connected in 
series. In this arrangement 
there will always be two 
paths in parallel through the 
armature, regardless of the 
number of poles. Armatures 
wound with this kind of a winding are called two-circuit, or 
series-wound, armatures. Only one set of brushes is required, 
but a maximum of ^p -^ 2) sets can be used. 

In calculating the e.m.f. that will be developed in a wind- 
ing of this kind, equation (125) can be used and (b) will 
always be equal to 2. The armatures of the machines are 
usually series wound when their output is to be at a high 
voltage and low current, while if the output is to be at a low 
voltage and large current, they are parallel wound. 

245. Types of Drum Windings. — Drum windings (closed- 
coil) are of two kinds: 

(a) Lap windings. 

(b) Wave windings. 

(a) A simple (simplex) lap winding is shown in Fig. 195 
and a developed form of the same winding is shown in Fig. 
196. It will be observed that the front and the back pitches 
differ in sign, or the winding laps back upon itself. The num- 
ber of paths (b) in parallel through an armature wound with 
a simple (simplex) lap winding is equal to the number of 
poles (p). The number of sets of brushes required in oi'der 
that all the inductors be effective in producing a current in 



AEMATUEES 



331 



the external circuit is equal to (p). Lap windings are usu- 
ally used on armatures whose output is at a low voltage 
and large current. 

(b) A simple (simplex) wave winding is shown in Figs. 
197 and 198. It will be observed that both the front and the 
back pitches are of the same sign and the winding advances 
around the armature in the form of waves. There will be 
only two paths through an armature wound with a simple 
(simplex) wave winding regardless of the number of poles. 
Only two sets of brushes are required for such a winding, 




but as many as (p) sets can be used. Wave windings are 
usually used on armatures whose output is at a high volt' 
age and low current. 

If the front and the back pitches in a wave winding be so 
chosen that in starting with a commutator segment and pass- 
ing through an element of the winding in a clockwise direc- 
tion you arrive at a segment to the right of the one from 
which you started, the winding is said to be progressive; if 
the segment is to the left of the one from which you started 
the winding is said to be retrogressive. 

If the front and back pitches in a wave winding be so 
chosen that in starting with a given commutator segment 
and, after passing in a clockwise direction through as many 
elements of the winding as there are pairs of poles, you 
arrive at a segment to the right of the one from which you 
started, the winding is said to be progressive; and if the seg- 
ments be to the left of the one from which you started, 
the winding is said to be retrogressive. 

When the average pitch of a winding differs considerably 



232 



PEACTICAL APPLIED ELECTEICITY 



from the number of inductors (Z) divided by the poles (p), 
the winding is called a chord winding. 

246. Multiplex Windings. — 
Armatures may be wound 
with two or more independent 
windings, the inductors and 
the commutator segments of 
the two windings being sand- 
wiched between each other, 
as shown in Fig. 199. A 
winding composed of two in- 
dependent windings is called 
a duplex winding; one com- 
posed of three independent 
windings is called a triplex 
winding, etc. The brushes 
must, of course, be increased 
in width, if the commutator segments remain the same in 
breadth when a multiplex winding is used, in order that the 
current may be collected from all the various windings at 
the same time. 

247. Re-Entrancy. — In section (238) it was stated that a 
winding which closes upon itself is called a closed-circuit wind- 
ing. Such a winding is also known as a re-entrant winding, 
because it re-enters upon itself. A winding is said to be singly 




Fig, 197 




Fig. 198 



re-entrant when the entire winding must be traced through 
before you return to the inductor from which you started. 
If only one-half of the winding need be traced out before the 
inductor from which you started is again reached, the wind- 
ing is said to be doubly re-entrant. Likewise only one-third 
of the winding is passed through in a triply re-entrant wind- 



ARMATURES 



233 




mg. 199 



ing before you return to the conductor from which you started, 
and so on for multiply re-entrant windings. 

The term re-entrancy as 
used in the remainder of the 
chapter has a meaning en- 
tirely different than that just 
given, it being used as a 
factor in determining the 
number of circuits between 
positive and negative brushes. 
In the above definition of re- 
entrancy, there would be no 
difference between the multi- 
plicity and the re-entrancy of 
a winding. A simplex or 
multiplex winding, however, 
may be singly or multiply re- 
entrant, and the re-entrancy 

in the case of a lap winding is equal to the total number of 
paths through the armature divided by the product of the 
number of poles and the multiplicity of the winding, and in 

the case of the wave winding 
it is equal tc the total num- 
ber of- paths divided by twice 
the multiplicity of the wind- 
ing. The winding shown in 
Fig. 200 is simplex doubly re-^ 
entrant, and the one shown in 
Fig. 199 is duplex singly re- 
entrant. The number of paths 
through the winding is the 
same in both cases. 

248. Number of Paths 
Through Armature Windings. 
— As stated in section (244), 
the number of paths in par- 
allel (b) for a simplex spirally-wound ring is (p), and for a 
simplex series-wound ring (2). When such windings are made 
multiplex, the number of paths is increased, the number in 
I the second case being equal to the original number multiplied 
by the multiplicity of the winding. Thus a triplex series- 




Fig. 200 



234 PBACTICAL APPLIED ELECTRICITY 

wound ring winding would have (3X2) paths in parallel, and 
a duplex spirally-wound ring winding would have (2Xp) paths 
in parallel. Winding tables are given in Chapter 20, which 
show the relation between the multiplicity, the re-entrancy, 
the number of paths, the number of inductors in series in 
each path, etc. 

A closed-coil drum winding must satisfy the following con- 
ditions. No drum winding can have an odd number of in 
ductors, for that would be equivalent to not having a whol^ 
number of turns. The even-numbered inductors may be re 
garded as the returns for the odd-numbered inductors, or 
\ice versa, and both the front and back pitches must always 
be odd in simplex windings in order that you pass from an 
odd to an even numbered inductor at one end of the arma 
ture and from an even to an odd numbered inductor at the 
other end in tracing out the winding. 

The front and back pitches should be approximately equal 
and correspond in value to (Z -f- p) in order that the in- 
ductors connected in series and moving under poles of oppo- 
site polarity have their e.m.f.'s additive. 

249. Choice of Front and Back Pitch for a Given Number 
of Inductors. — (A) Lap and wave windings in general. 

(a) All the elements or coils composing the winding must 
be similar, both mechanically and electrically, and must be 
arranged symmetrically • with respect to each other and the 
armature core. 

(b) In a simplex winding each inductor must be encoun- 
tered only once and the winding must be re-entrant. 

(c) If the winding is multiplex, each of the simplex wind- 
ings composing it must fulfill condition (b). 

(B) Choice of front and back pitch for lap windings. 

(a) Front and back pitches must be opposite in sign. 

(b) The front and back pitches must be numerically un- 
equal, for if they were equal the coil would be short-circuited 
upon itself. 

(c) The front and back pitches must differ by 2 in th 
case of a simplex lap winding; that is, 

Yb -f Yf = ± 2 

(d) The front and back pitches differ by 2 X in the casp 



ARMATURES 235 

of a multiplex winding, (X) being the number of independent 
simplex windings composing the multiplex winding. 

(e) The number of inductors (Z) must be an even num- 
ber. If the winaing is placed on a slotted armature the 
number of inductors must be a multiple of the number oi 
slots. The number of slots may be even or odd. 

(C) Choice of front and back pitches for wave windings. 

(a) The front and back pitches must be alike in sign, and 
odd. 

(b) The front and back pitches may be equal, or they 
may differ by two or any multiple of two. It is customary 
to make them nearly equal to (Z-v-p). 

(c) The number of inductors (Z) for a simplex winding 
should comply with the equation 

Z == P Yav ± 2 

(d) And for a multiple winding: 

Z = p Yav ± 2 X 

250. Armature Winding Table. — Tables I, J, and K in Chap- 
ter ^0 give the values of the various (quantities for the differ- 
ent types of armature windings. The symbols used in these 
tables represent the following quantities: 

Z = the total number of inductors on the armature 

X = the number of independent windings 

b = the total number of paths through the armature 

bi = the number of paths in parallel in each winding 

m = the field steps 

y = the resultant pitch 

Yk= the commutator pitch 

Yf = the front pitch 

Yb = the back pitch 

K = the number of commutator segments 

g = the number of inductors per group 

G = the total number of groups 
Yav = the average pitch 

Note. — The tables given in Chapter 20 were made up by the 
use of equations given in ^'Design of Dynamos" by S. P. 
Thompson. 

251. Equi potential Connections. — The e.m.f.'s generated in 
the various paths that are connected in parallel in any arma- 
ture winding will, as a rule, differ in value due to inequali- 



236 PRACTICAL APPLIED ELECTRICITY 

ties in winding, location in magnetic field, armature out 
of center, etc. This difference in e.m.f. in the various paths 
will result in unequal currents in the different paths through 
the armature. In order to reduce this unbalanced condi- 
tion as much as possible the points in the winding that are 
supposed to be at the same potential are connected by heavy 
copper leads, called equipotential connections. 



CHAPTEK XII 

STORAGE BATTERIES, THEIR APPLICATIONS AND 
MANAGEMENT 

252. The Storage Cell. — A storage cell, secondary cell, oi 
accumulator, as it is variously called, is a voltaic cell in 
which a chemical action is first produced by an electrical cur- 
rent through the cell from some external source of energy, 
after which the cell is capable of delivering a current to an 
external circuit by means of a secondary or reversed chem- 
ical action. The process of storing the electrical energy by 
sending a current through the cell from some external source 
of energy is called charging. When the cell is producing a 
current in an external circuit or it is supplying energy it is 
said to be discharging. ^ 

In charging a storage cell a larger voltage is required be- 
tween the terminals of the cell than the cell is capable of pro- 
ducing on discharge, on account of the internal resistance of 
the cell and an action on the surface of the plates similar to 
polarization in the primary cell. The (IR) drop in the cell 
opposes the free flow of the electricity through the ceil, both 
on discharge and charge, and the energy the cell is capable 
of supplying on discharge is less than the energy input to 
the cell when it is being charged. No storage cell, as a re- 
sult, can have an efficiency of 100 per cent, the efficiency 
decreasing with an increase in internal resistance. The 
greater the internal resistance the greater the variation in 
the value of the terminal voltage with a change in load. 

253. Types of Storage Cells. — Storage cells may be divided 
into two main groups, according to the kind of materials 
used in the construction of the plates, viz, lead storage cells 
('sections 254 to 274 inclusive), and non-lead storage cells 
(sections 275 to 278 inclusive). 

254. Lead Storage Cells. — In the construction of the lead 
storage battery, the cathode is made of lead peroxide (Pb02) 
and the anode of spongy metallic lead. These two plates, 

237 



338 PEACTICAL APPLIED ELECTRICITY 

or "grids," as they are called, are immersed in an electro- 
lyte of dilute sulphuric acid. The lead peroxide and spongy 
lead are called the active materials of the cell. They are 
both converted in lead sulphate (PbS04), which is insoluble, 
when the cell is being discharged, and this lead sulphate 
is reconverted into lead peroxide and spongy lead when the 
cell is being charged. 

The electrode of a storage cell, which is the cathode on 
discharge, is called the positive grid and the other electrode 
is called the negative grid. 

255. Action of a Lead Storage Cell While Discharging. — 
In discharging a lead cell, the electrolyte (H2SO4) is split up 
by the current into hydrogen (H) and sulphion (SO4). The 
hydrogen which is liberated at the cathode converts the 
lead peroxide (Pb02) into lead oxide (PbO). The lead oxide 
immediately combines with a part of the electrolyte (H2SO4), 
forming lead sulphate and water. Lead- sulphate (PbS04) 
is formed at the anode by the sulphion (SO4), combining 
with the spongy lead (Pb). 

The lead sulphate which is formed during the discharging 
process is more bulky than the active materials themselves 
and, as a result, there is an expansion in the plates of the 
ceil. There is also a decrease in the density of the elec- 
trolyte on account of the absorption of the sulphion (SO4) 
by the active material. 

The chemical action taking place in the cell can be easily 
shown by the equation 

Discharging (current from negative to positive grid) 

Positive grid Pb02 + H2SO4 + H2 = 2H2O + PbS04 

Negative grid Pb + SO4 = PbS04 

256. Action of Storage Cell While Charging. — In charging 
a lead storage cell, an action takes place which is just the 
reverse of that described in section (255). The lead sul- 
phate (PbS04) on one grid is converted back to lead per- 
oxide (Pb02), the lead sulphate (PbS04) on the other grid is 
converted back to spongy lead (Pb), the density of the elec- 
trolyte increases and the volume of the active material 
decreases. The following equations will serve to show the 
chemical action that takes place in the cell when it is being 
charged. 

Charging (current from positive to negative grid) 



m 



STORAGE BATTERIES 



239 



Positive grid PbS04 + 2H2O + SO4 = 2H2SO4 + PbOo 
Negative grid PbS04 + H2 = H2SO4 + Pb 

257. Storage Battery Grids. — Two general processes are 
employed in the manufacture of storage battery grids, viz, 
the Plante process and the Faure process. 

258. The Plante Process. — In the Plante process the 
surface of a lead plate is subjected to the action of a suitable 
acid which converts the surface of the plate into active mate- 
rial. In the first process the lead plates were exposed to the 
action of dilute sulphuric acid, and the action was accelerated 
by making the plates being formed alternately the anode and 
cathode of the electrolytic cell. The modern process is far 
superior to the old method, the formation of the active mate- 
rial being brought about in a much shorter time by adding a 
small quantity of lead dissolving acid, such as acetic or nitric 
acid, and increasing the area of the plates exposed to the 
action of the acid, by cutting grooves in their surface. 

259. Faure Process. — In the 
Faure process the active ma- 
terial is manufactured in bulk 
and introduced by a mechan- 
ical process into openings in 
the lead grids. The original 
process, and one that is used 
quite extensively at the pres- 
ent time, consisted in mixing 
litharge or a mixture of lith- 
arge and red lead with dilute 
sulphuric acid, and placing 
the paste in suitable support- 
ing frames, the combination 
forming the grids. 

260. Examples of Plante and Faure Plates. — The grids 
manufactured by the Gould Storage Battery Company, as 
shown in Fig. 201, are a good example of the modern Plante 
plate. Thick plates of lead of the proper dimensions are passed 
over rapidly revolving rolls which consist of a large number 
of thin disks separated from each other by thin washers. The 
action of these disks is such that fins of lead are raised upon 
the surface of the lead plates. Both surfaces of the plates are 
subjected to the action of the spinning rolls, but central webs 




Fig. 201 



240 



PEACTICAL APPLIED ELECTRICITY 




Fig. 202 



and cross-ribs are left where the plate is in no way changed. 

The purpose of these central webs and cross-ribs is to add 

conductivity and strength to the plates. 

The positive grid used in the sta- 
tionary type of storage cell manu- 
factured by the Electrip Storage 
Battery Company, which are often 
called "chloride" cells, is shown in 
Fig. 202. A casting is first made 
of lead-antimony alloy, with numer- 
ous holes in it, and small coils of 
pure lead are placed in these holes. 
These coils of lead are then con- 
verted into active material by the 
Plante process. 

The 'negative grid used in the 
stationary storage cell of the Elec- 
tric Storage Battery Company is 
shown in Fig. 203. Two plates of 
pure lead with numerous holes in 

them are pressed together over a block of active material 

which squeezes out through the holes. While the plates are 

still under pressure they are riveted together. In the "exide'* 

cell made by the Electric Storage 

Battery Company, the grids are 

cast of a lead-antimony alloy with 

a large number of small openings 

in them. The openings in the 

plates that are to form the positive 

grids are filled with a paste of red 

lead and dilute sulphuric acid, and 

the openings in the plates that are 

to form the negative grids are filled 

with a paste of litharge and dilute 

sulphuric acid. 
261. Comparison of Plante and 

Faure Plates. — The Plante plates 

are more costly for a given output 

Fig" '^OS 
than the Faure plates. They are " 

more bulky, heavier, and more easily injured by impurities in 

the electrolyte. They are, however, able to stand a more 




STORAGE BATTERIES 



241 



rapid charging and discharging without injury. They do not 
lose their active material as easily as the Faure plates. They 
are in general more durable and dependable than the pasted 
plates and have longer life. The Faure plates are cheap, 
light, and occupy a small space. They are not so easily dam- 
aged by impurities in the electrolyte; but their efficiency is 
less than the Plante plates for higher rates of discharge. 

262. Capacity of a Storage Cell. — The unit in which the 
capacity of a storage battery is measured is the ampere-hour, 
and the capacity is usually based upon the eight-hour dis- 
charge rate. As an example, a 200-ampere-hour battery would 
deliver a current of 25 amperes continuously for 8 hours. 
Theoretically this same battery should deliver a current of 
50 amperes for 4 hours, or a current of 12 amperes for 16 
hours; but as a matter of fact the ampere-hour capacity of a 
battery decreases with an increase in the rate of discharge. 
The change in the capacity of a battery due to a change in 
the discharge rate is given in Table No. IX. The 8-hour rate 
is taken as a basis in figuring the output at other rates. 

TABLE NO. IX 

PERCENTAGE VARIATION OF THE CAPACITY OF A STORAGE 

BATTERY DUE TO A CHANGE IN THE RATE 

OF DISCHARGE 





Percentage of capacity at S^hour rate 








Plante (-f) 


Rate in hours 


Plante 


Faure 


Faure (— ) 


8 


100.0 


100.0 


100.0 


7 


99.0 


96.0 


97.0 


6 


96.5 


92.0 


93.5 


5 


93.0 


86.0 


89.0 


4 


88.0 


80.0 


83.0 


3 


80.0 


72.0 


75.0 


2 


70.0 


61.0 


65.0 


1 


55.0 


40.0 


50.0 



263. The Electrolyte. — The electrolyte should be made of 
sulphuric acid made of sulphur and not from pyrites. Acid 
made from pyrites contains some iron and the presence of 
this metal in the electrolyte is injurious to the plates. It is 
not essential that the electrolyte be chemically pure, but it 
should not contain any chlorine, nitrates, arsenic, mercury, 
copper, platinum, nitric or acetic acid. The electrolyte should 



242 



PRACTICAL APPLIED ELECTRICITY 



always be purchased from a very reliable chemical company 
that will guarantee its being free from impurities which will 
be injurious to the plates in the storage battery. 

The acid may be purchased diluted until the specific gravity 
is of the desired value for immediate use in the cell. In 
Gome cases, however, it may be desirable to purchase the acid 
and reduce its specific gravity at the point where it is to be 
used. Only distilled or rain water should be used in diluting 
the acid and the acid should always be poured into the water. 
There will be considerable heat liberated and the solution 
should be allowed to cool before a determination of the 
specific gravity is made as there is quite a change in the 
specific gravity due to a change in the temperature of the 
electrolyte. The density of the acid to use will of course 
depend upon the kind of cell it is to be used in, the ampere- 
hour capacity of the cell, its rate of discharge and charge, etc. 
The density that will give the best results for any particular 
cell is, in the majority of cases, specified by the makers. 

264. Density and the Hydrometer. — The density 
of any substance is the ratio of the weights of 
equal volumes of the substance and water. Thus, 
if the specific gravity of a certain quantity of 
sulphuric acid is 1.25, it means that a certain 
volume of the acid will weigh 1.25 times as much 
as ihe same volume of pure water. 

The hydrometer is an instrument for measuring 
the density of liquid. It is placed in the liquid and 
is so constructed that a portion of it projects above 
the surface of the liquid. The amount projecting 
Fig. 204 will vary with the density of the liquid and a 
suitable scale attached or marked on the up- 
wardly projecting portion will afford a means of determining 
the density of the liquid direct from the scale reading. The 
reading is taken at the surface of the liquid. The outline of 
a hydrometer is shown in Fig. 204. A small quantity of lead 
shot in the lower portion gives the instrument the required 
weight and serves to hold it always in an upright position. 

265. Containing Vessel and Separators. — There are a num- 
ber of different kinds of containing vessels for the plates and 
electrolyte of a storage cell, and perhaps the most important 
of these are those made of rubber, glass, and lead-lined 



STORAGE BATTERIES 



243 



wooden tanks. The rubber containing vessel Is used In the 
construction of portable cells because it will stand more abuse 
than glass and is much lighter. Glass containing vessels are 
used for stationary cells of moderate capacity. The lead-lined 
wooden tanks are used for very large cells as they are 
stronger than either the glass or the rubber and the increase 
in weight makes no great difference. 

When the plates are placed in 
the containing vessels, they are 
positive and negative alternately. 
This results in adjacent plates be- I 
ing of opposite polarity and some | 
means must be provided for sepa- 
rating them. Perforated rubber 
sheets are used in the smaller 
cells, rubber sheets and thin 
sheets of porous wood in medium 
size cells, and glass rods in the 
larger cells. The interior of a 
cell is shown in Fig. 205. 

266. Sulphation. — In the section 
describing the chemical action of 
the storage battery, it was men- 
tioned that lead sulphate was 
formed when the cell was dis- 
charged. This lead sulphate is 
insoluble in sulphuric acid and oc- 
cupies a larger volume than the 
pure lead or lead peroxide which 
forms it, and it is a non-conductor. 

In discharging a storage battery, it should not be allowed to 
go beyond a point where a small part of the active material 
is converted into lead sulphate. If the battery be over dis- 
charged there will be an excessive amount of the sulphate 
formed or as it is termed, an oversulphation. 

The presence of the excessive amount of sulphate results 
in the surface of the plates being covered with white crystals 
and the pores of the plates are closed up due to the increase 
In volume. The surface of the active material exposed is, as a 
result, decreased due to the presence of the crystals, and the 
increase in volume will cause the active material to be loos- 




Fig. 205 



244 PEACTICAL APPLIED ELECTEICITY 

ened from the grid, or it may result in the plate being bent 
out of shape, which will result, in some cases, in the plates 
being fractured and the active material falling to the bottom 
of the cell as sediment. The distortion of the plates is known 
as buckling. A high discharge rate results in a greater 
expansion than a low rate and the active material is more 
likely to fall away when the battery is being discharged at a 
high rate than it is when discharged at a low rate. 

267. Battery Troubles and Their Remedies. — The following 
troubles are perhaps the most important ones encountered in 
the operation of a storage battery: 

(A) Loss of capacity. 

(B) Loss of voltage. 

(C) Corrosion of electrodes. 

(D) Buckling of plates. 

(E) Shedding active material. 

268. (A) Loss of Capacity.— ^The loss of capacity may be 
due to any one or all of the following causes: (a) sulpha- 
tion; (b) loss of active material; (c) loss of electrolyte; (d) 
sulphate between grid and the active material; and (e) con- 
traction of active material on spongy lead plate. 

(a) The causes of sulphation have already been discussed 
but the means of treating sulphated plates will be given here. 
To remove the sulphate, charge the battery at as high a rate as 
possible without causing the temperature of the cell to exceed 
110° Fahrenheit until the plates gas very freely, then reduce 
the rate to the normal 8-hour rate and continue until the 
plates again begin to gas. Then reduce to half the 8-hour 
rate and continue until the plates gas. Now give the battery 
a partial discharge and then recharge as just described. This 
cycle of operations may have to be repeated a number of times 
before all the sulphate is removed. 

(b) Loss of active material is usually due to the plates not 
being worked under normal conditions; if this is not the case, 
the, plates are poorly designed. This loss is due chiefly to 
rapid charge and discharge, sulphation, and a long over- 
charge. 

(c) Loss of electrolyte due to evaporation may cause the 
surface of the liquid to fall below the upper edge of the plates. 
The remedy is obvious. 

(d) In the Faure type of plates, sulphate may form between 



STORAGE BATTERIES 245 

the active material and the supporting grid, which greatly 
increases the electrical resistance of the cell and decreases 
the area of the active material. 

(e) The active material on the spongy-lead electrode will 
contract and such shrinkage will close up the pores and 
greatly reduce the active surface. There are methods of 
preventing this contraction in use by several companies. If 
you think your battery trouble is due to this cause, it would 
be best for you to get advice from the manufacturers of the 
battery. 

269. (B) Loss of Voltage. — The causes of loss in voltage 
are the same as those resulting in a loss of capacity and the 
treatment of the cell for the two conditions is identical. 

270. (C) Corrosion of Plates. — The corrosion of the plates 
is usually due to the presence of some injurious impurities in 
the electrolyte and the only remedy is to change the electrolyte. 

271. (D) Buckling of Plates. — Any condition of operation 
of the cell which will result in an unequal chemical action 
over the surface of the plates will result in an unequal 
expansion of the active material and hence, a tendency for 
the plate to expand more in some spots than others. These 
stresses which are set up in the plates relieve themselves by 
changing the form of the plates. Buckling is usually due to 
improper design and cannot be overcome except by recon- 
struction. 

272. (E) Shedding of Active Material. — When the active 
material on the plates drops off more rapidly than it should — 

. which is indicated by the rapid accumulation of sediment in 
the bottom of the cell — the cell should be worked at a low 
rate of discharge, never being discharged below 1.75 volts 
and never being charged above 2.4 volts. 

273. Management of a Storage Battery. — There are a few 
important rules that one should always bear in mind in the 
care of a lead storage battery. 

(a) Always use a pure electrolyte. 

(b) Never allow the surface of the electrolyte to fall below 
the upper edge of the plates. 

(c) Always maintain the specific gravity at the value speci- 
fied by the manufacturers. 

(d) Keep cells well cleaned, never allow sediment to rise 
until it is in contact with the lower edge of the plates. 



246 PEACTICAL APPLIED ELECTRICITY 

(e) Keep all separators in place so that there is no danger 
of the plates coming in contact with each other and shorting 
the cell. 

(f ) Keep cells well insulated by placing them in trays 
that are supported on insulators. 

(g) Inspect all cells occasionally for leaks in the contain- 
ing vessel and either replace the broken vessel by a new one 
or repair it immediately. 

(h) Always charge the battery as soon as possible after 
discharge. 

(i) Never overcharge the battery or after the negative 
plates begin to gas, except as stated in section (268). 

(j) Never allow the battery to discharge below 1.75 volts 
at the normal rate of discharge and 1.6 volts when discharg- 
ing at the 1-hour rate. 

(k) Watch the plates for signs of sulphation and if any 
sulphate appears, treat them immediately as described in 
section (268). 

(I) Always keep the temperature of the battery below 110° 
Fahrenheit. 

(m) Give the battery a prolonged over-charge occasionally 
until free gassing of the negative plate has continued for one 
hour. 

274. To Put a Cell Out of Service. — When a cell is to be 
out of service for several months or for an indefinite period, it 
can be treated as follows and will then need no attention until 
it is desired to be used again. Fully charge the cell, then 
discharge it for about two ho.urs at the normal rate. After the 
cell has been discharged, draw off the electrolyte and fill the 
vessel with distilled water. Now again discharge the cell at 
the normal rate until its terminals must be shorted to produce 
the discharge current. Pour out the water and again fill the 
cells, allowing them to stand for a day or so when this water 
should be drawn off and the plates allowed to dry. To put the 
cell back in service, the electrolyte should be placed in the 
cell and the cell should be given a prolonged over-charge. 

275. Non-Lead Storage Cells.^ — Almost any primary cell may 
be made to act more or less as a secondary cell; as, for 
example, the gravity cell may be charged by passing a current 
through it opposite to the direction the current passes through 
the cell on discharge. Quite a number of cells have been 



I 



STOKAGE BATTERIES 247 

devised in which metals other than lead is used for one or both 
of the plates. Reynier made a cell in which the negative plate 
was composed of zinc instead of lead, and this zinc was con- 
verted into zinc sulphate when the cell was discharging, which 
dissolved in the electrolyte. The electromotive force of this 
cell was considerably higher than that of the ordinary lead 
cell and it was quite a bit lighter, since for the storage of a 
given amount of energy the weight of zinc required is much 
less than that of the equivalent lead. This cell, however, is 
not very satisfactory in operation, since there are trees of zinc 
formed on the negative plate during the process of charging, 
and these trees are likely to extend out from the negative 
plate a sufficient distance to short-circuit the cell. There is 
also a difference in the density of the electrolyte at the top 
and the bottom of the cell, which results in the action on the 
plates not being uniform. This difficulty was overcome to a 
certain extent by placing the plates in a horizontal instead of 
in a vertical position. Where the plates are horizontal, however, 
there is an accumulation of the liberated gases upon the sur- 
face of the plates and, as a result, an increase in the internal 
resistance of the cell. 

Waddell and Entz constructed a storage cell in which 
copper and zinc are used as the plates and the electrolyte is 
an alkali solution. When the cell is discharged, the positive 
plate consists of porous copper and on charging, the elec- 
trolyte is decomposed, metallic zinc is deposited on the 
negative plate, the porous copper forming the positive plate is 
oxidized, and the liquid forming the electrolyte is converted 
into a solution of caustic potash. The electromotive force of 
this cell is very low, it being only about .7 volt and, as a 
result, approximately three times as many cells must be used 
to give a given voltage as would be required if the lead cells 
were used. This is a very serious objection to this particular 
cell or to any other low voltage cell. 

276. Edison Storage Battery. — In the Edison battery the 
active materials are oxides of nickel and iron, respectively, in 
the positive and the negative electrodes, the electrolyte being 
a solution of caustic potash in water. The retaining vessels 
for these cells are made from sheet steel, their walls being 
corrugated to add strength with a minimum weight. The 
completed can is nickel-plated, which protects the steel from 



248 



PEACTICAL APPLIED EL.ECTKICITY 



rust and at the same time adds to the appearance of the cell. 
There are a number of different types of Edison cells on the 
market, but they differ only in the number of plates they 
contain. Each positive plate consists of a grid of nickel-plated 
steel holding 30 tubes filled with the active material, in two 
rows of 15 tubes each, as shown in Fig. 206. The tubes are 
made of very thin sheet steel, perforated and nickel-plated. 
Each tube is reinforced and protected by small ferrules, eight 
in number. These ferrules prevent expansion and thereby 
retain perfect internal contact with the active material at all 
times. The active material in the tubes is interspersed with 
thin layers of pure metallic nickel in the form of leaves or 
flakes. The pure nickel flake that is used is manufactured 
by a special electrochemical process. 

Each negative plate comprises 24 
f^^^^w. «. .^.^^^^^^^^^^^^^ ^^^ rectangular pockets supported 

in three horizontal rows in nickel- 
plated steel grids, as shown in Fig. 
-. ; 0-, 207. The pockets are made of thin 

'i u y nickel-plated steel, perforated with 

H fine holes, each pocket being filled 

with an oxide of iron very similar 
to what is called iron rust. In the 
construction of the negative plate 
each pocket is subjected to a very 
high pressure, so that it becomes 
practically integral with the sup- 
porting grid. 

The positive and the negative 
plates are hung alternately on two 
connecting rods, the positive plates 
being electrically connected to one 
rod and the negative plates to the other. The plates are 
properly distanced on these rods by nickel-plated steel spac- 
ing washers, and ^re held firmly in place and contact by 
nuts screwed on both ends. In assembling the plates a 
specially shaped lug is placed in the center of each of the 
connecting rods, as shown in Fig. 207. These rods project 
upward and are of suflficient length to protrude through open- 
ings in the top of the containing can, thus forming the ter- 
minals of the cell. There is always one more negative plate 




Fig. 206 



J 



STOEAGE BATTERIES 



249 



in the Edison cell than there are positive, just as there is in 
the lead cell, which results in both outside plates being nega- 
tive. Special insulators are used in separating the plates and 
preventing them from coming into contact with the containing 
can. 

277. Chemistry of the Edison 
Storage Battery. — Starting with the 
oxide of iron in the negative, green 
nickel hydrate in the positive, and 
potassium hydrate in solution, the 
first charging of a cell reduces the 
iron oxide to metallic iron while con- 
verting the nickel hydrate to a very 
high oxide, black in color. On dis- 
charge, the metallic iron goes back 
to iron oxide and the high nickel 
oxide goes to a lower oxide but not 
to its original form of green hydrate. 
On every cycle thereafter, the nega- 
tive charges to metallic iron and dis- 
charges to iron oxide, while the posi- 
tive charges to a high nickel oxide. 
Current passing in either direction 
(charge or discharge) decomposes 
the potassium hydrate of the elec- 
trolyte, and the oxidation and reduc- 
tions at the electrodes are brought 
about by the action of its elements. 
An amount of potassium hydrate 
equal to that decomposed is always , 

re-formed at one of the electrodes by a secondary chemical 
reaction, and consequently there is none of it lost and its 
density remains constant. 

The eventual result of charging, therefore, is a transference 
of oxygen from the iron to the nickel electrode, and that of 
discharging is a transference back again. This is why the 
**Edison" is sometimes called an oxygen lift cell. 

When both electrodes become fully charged, the elements 
of the decomposed potassium hydrate can no longer act on 
them, but instead they react to produce hydrogen and oxy- 
gen — the elements of water which are given off as a gas. 




Fig. 207 



250 PRACTICAL APPLIED ELECTRICITY 

Nickel plates in the positive and mercury in the negative 
do not take part in the chemical reaction but are used solely 
to bring the particles of active material into good electrical 
contact with the conducting support. 

278. Care of an Edison Storage Battery. — Edison storage 
batteries are shipped in a discharged condition, and should 
be given a long initial charge before being put into service. 
The duration of this first charge should be about 15 hours at 
the normal rate, and it is recommended by the company that 
this overcharge be repeated once every two weeks for the 
first two months, when the battery is in constant service. If, 
however, the battery is not in constant service, it should be 
given a long charge after every 12 complete charges and dis- 
charges until five long charges have been put in. This con- 
stitutes what might be called the forming process, and it is 
advisable to repeat the long charges about every two months 
throughout the life of the battery. 

In charging an Edison battery, the seven-hour charge at the 
normal rate is arbitrarily chosen as normal for the reason 
that a battery will retain good efiiciency up to this point. 
This rate, however, is not fixed and may be varied at will, the 
highest practical output being reached on a ten-hour charge, 
which gives a maximum efficiency of about 30 per cent above 
normal rated output. Overcharging is in no way harmful, 
provided the temperature is not allowed to exceed 100° Fahren- 
heit at any time while charging. The celjis must be frequently 
filled when they are overcharged and overcharging means a 
waste of energy. The evaporation must be replaced with pure 
distilled water, and under no circumstances use any acid. The 
solution should never be allowed to go below the tops of the 
plates, the proper height being one-half inch above the plates. 
It is always advisable to fill the battery before charge as in 
the process of charging the liquid is raised to a false level. 

The electrolyte — which is a 21 per cent solution of potash 
(KOH) — in a battery in constant service, being charged and 
discharged each day, should be renewed once every eight or 
nine months. 

279. Connmercial Applications of Storage Batteries. — The 
following list gives the principal uses to which the storage 
battery may be placed: 

(a) To supply energy to portable electrical apparatus. 



STORAGE BATTERIES 251 

(b) As a source of constant potential and current in elec- 
trical laboratories. 

(c) As a source of energy in telephone and telegraph work. 

(d) To supply energy for train-lighting. 

(e) To reduce the fluctuations in the load on a generator, 
by operating the battery and the generator in parallel. 

(f) To supply energy during certain hours when the load 
on a power plant is low and thus allow the generator to be 
shut down. 

(g) To supply energy and aid the generator in carrying the 
maximum load (peak) which usually lasts only a few hours. 

(h) To change from a high to a low voltage by charging 
the cells in series and discharging them in parallel, or vice 
versa. 

(1) As a means of subdividing the voltage of a main gen- 
erator, enabling it to supply energy to a multi-voltage system. 

(j) To supply energy for electrically driven vehicles and 
boats. 

280. Portable Storage Batteries. — Practically all the bat- 
tery companies manufacture a portable form of storage cell. 
These cells are usually constructed so that they will be as 
light and compact as possible, and mounted in a suitable 
containing case outside the one holding the electrolyte, which 
gives ample protection to the cell and affords a means of 
easily carrying the cell, by providing this outer case with 
some kind of a handle. The number of cells mounted in each 
outer containing case may vary depending upon the particular 
use to which the battery is to be placed. 

The great objection to portable storage cells is their great 
weight and this is almost prohibitive of their general use. 
Portable cells are used in operating small lighting systems 
such as occur on automobiles and launches, in operating spark 
coils for gas engines, in operating small lamps and motors for 
various special purposes, etc. 

281. Storage Batteries in Electrical Laboratories. — The 
storage battery is a valuable source of energy in electrical 
laboratories, as the regulation of the voltage and the current 
can be accomplished by means of the adjustment of a simple 
rheostat of some kind. The voltage of the battery can be 
changed by changing the grouping of the cells, a maximum 
voltage being obtained when they are all connected in series 



352 PRACTICAL APPLIED ELECTRICITY 

and a minimum voltage when they are all connected in 
parallel. , When a large current is wanted at a low voltage 
it can be more easily obtained and at a less expenditure of 
energy than if the same current were taken from a higher 
voltage. The connections of a charging and discharging board 

are shown in Fig. 209. v»/„„u 

282 Storage Batteries in Telephone and Telegraph Work. 
-Storage batteries have come into general use in telephone 
exchanges since the introduction of what is known as the 
common-battery telephone system. In this system instead 
of each subscriber having a battery of two or more cells 
placed in his instrument, these batteries are all combined in 
one large battery located at the central office, and each sub- 
scriber's line is supplied with current from this main or 
central battery. The storage battery is preferable for this 
main battery for the following reasons: Lower first cost; 
smaller space required; lower internal resistance: more con- 
stant electromotive force; rapidity of recharge, and low cost 
of maintenance. Two batteries are usually placed m each 
exchange so that one can be on charge and held in reserve 
while the other battery is in use. 

In telegraph work the storage battery may be used alone 
or in combination with small generators in supplying current 
to operate the telegraph instruments. _ 

283. Storage Batteries for Train Lighting.-The simplest 
method of lighting a train by electricity is to install a sma 
dynamo on the locomotive, which may be driven by a small 
steam engine or turbine. With this arrangement there is the 
objection that the cars must be illuminated by some other 
means when the locomotive is uncoupled. This has led to the 
storage-battery system of supplying energy to each car sepa- 
rately even though it be disconnected from the main tram. 
The battery is so connected that it is being charged when 
connected to the main generator leads, provided the voltage 
between the generator .leads is greater than the terminal 
voltage of the battery. When the generator voltage drops 
below a certain point, due to any cause, the battery discharges 
and the lamps continue to burn just as though the generator 
were operating. The connections of the battery lamps and 
the generator are controlled by a specially constructed switch 
that is automatically operated. In some cases a small gen- 



STOEAGE BATTEEIES 253 

erator is mounted on each car and driven by a belt that passes 
over a pulley on one of the axles. The generator in this 
system is inoperative when the car is at rest and the battery 
must necessarily carry the load. 

284. Storage Battery and Generator in Parallel.— Often the 
load a generator is called upon to carry fluctuates in value 
and in a great many cases it is desired that the voltage 
between the lines remain as near constant in value as pos- 
sible. With a fluctuating load there would necessarily be a 
change in line voltage due to a change in the value of the 
copper drop unless there was a change in the voltage at the 

generator terminals that 
J. would counteract the cop- 

I + ^ per drop. By connecting 

"T" ^^ a battery in parallel with 

Batte ry ; Load the generator, as shown in 

-^C-. I Fig. 208, the battery 

-^ — ' charges when the load is 



Fig. 208 light and will discharge 

when the load is of such a 
value that the voltage between the line wires is less than the 
battery voltage. As a result, the copper drop in the leads 
from the generator remains nearer constant in value and 
there is a smaller fluctuation in the value of the line voltage 
due to a change in load than there would be if no battery 
were used. 

285. Storage Battery to Supply Energy During Certain 
Hours. — In a great many generating plants, the energy the 
generator is called upon to supply during certain periods is so 
small that it would be poor economy to operate the generator 
on so small a load. A storage battery can be installed of 
ample capacity to carry the load during this period and the 
plant entirely shut down. The output of a generating plant 
is represented by curve (A), Fig. 210. It will be seen from 
this curve that the load carried by the generator from about 
6 P.M. to 5 A.M. is very small. By installing a storage battery 
having the required k.w. hours' capacity, the generator can be 
shut down during part of the night. 

286. Storage Batteries to Aid Generators In Carrying the 
Maximum Load. — Assuming the load on a generating plant is 
similar to that shown by curve (B) in Fig. 210, sufficient 



254 



PEACTICAL APPLIED ELECTKICITY 



generator capacity must be installed to take care of the 
maximum load at any time without exceeding the allowable 
overload capacity of the generators. This would result in a 
large part of the equipment being practically idle during the 
greater portion of the day, or a part of the jnachinery would 
represent an investment from which there would be propor- 
tionally a small income. By installing a storage battery, the 



Amineter 



Battery 
Rhoestat 

Circuit 
BreaKer 




Voltmeter 

Voltmeter 
Switch 

-Series 
Parallel 
Switches 

Battery 
Switch 




HHHHHHHf-IHHHHHH 



Fig. 209 



generator capacity can be reduced, the battery being charged 
when the load on the plant is light and discharged in parallel 
with the generators when the load exceeds^ the capacity of 
the generators. The shaded portion of the curve in Fig. 210 
represents what is called the peak of the load. 

287. Storage Batteries Used in Changing Voltage. — The 
line voltage available in a great many cases is either too high 
or too low for a given purpose and some means must be 
employed for changing its value. An easy means of doing this 



STOEAGE BATTEEIES 



255 



is to use a number of storage cells and have them so arranged 
that they may be readily changed from any connection to an- 
other. Thus, if it is desired to reduce the voltage, the cells 



1 














1 




























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— 


■A 


^ 


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\^ 






^ 


% 


^ 












p 












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^ 


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■ \ 

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-d 












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fU 












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, 




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s, 
















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*s 


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ime in 


Hours 












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IE E 4 6 8 10 IE 2 4 6 8 10 IE 

Midnight Noon Mid;\4ght 

Fig. 210 



It x.tW 



of a battery may be charged in series and then connected in 
a number of groups and these groups in turn connected in 
parallel. If it is desired to raise the voltage, the cells should 
be charged in parallel and then connected in series for 
discharge. , 

288. S t o r a g e Batteries 

Used in Subdividing Voltage ^^ L, 

of a Generator. — The con- 
nections of a battery for di- 
viding the voltage of a gen- 
erator are shown in Fig. 
211. Supposing the genera- 
tor (G) is a 220-volt ma- 
chine and it is desired to Fig. 211 
obtain energy from it at 110 

volts. This can be accomplished by connecting the battery 
(B), which should have a voltage of about 220 volts, across 
the leads from the main generator and connecting a lead (N) 
to the center of the battery. With this connection there will 
'be a 110-volt pressure between each outside lead and the 
lead (N). 

289. Storage Batteries to Supply Energy for Electrically 
Driven Vehicles and Boats. — The storage battery is growing 




^T'^^ 



256 



PBACTICAL APPLIED ELECTKICITY 



in favor as a means of supplying energy to vehicles, such as 
pleasure automobiles, heavy trucks, and street cars; and it is 
also quite extensively used in small pleasure boats and sub- 
marines. The battery is connected to the motors through 




Fig. 212 



specially constructed controllers, so arranged that the cells 
are grouped to give a low impressed voltage on starting and 
as the controller is advanced to successive positions, the 
connections of the various cells are changed, all being con- 
nected in series when the controller handle is at its final 
position. A tray of six cells, as used in an electric automobile, 
is shown in Fig. 212. 



CHAPTEK XIII 

DISTRIBUTION AND OPERATION 

290. Systems of Distribution. — The power output of an 
electrical generator at any instant is equal to the product of 
the terminal voltage of the machine and the current supplied 
by the machine. With a change in output there must be a 
change in the value of this product and necessarily a change 
in the value of either the current or voltage, or both. When 
the power output of the generator changes due to a change 
in the value of the current, the machine is said to be a 

constant-voltage machine, 
and the method employed 
in supplying energy to the 
circuit connected to the ma- 
chine is called a constant- 
voltage, or parallel system 
of distribution. If the out- 
put of the machine changes 
due to a change in the ter- 
minal voltage, the current remaining constant, the method 
employed in supplying energy to the circuit connected to the 
machine is called constant-current, or series system of dis- 
tribution. 

291. Constant-Voltage Distribution. — In the constant-volt- 
age, or parallel system of distribution, the various devices that 
are being supplied with energy are connected in parallel 
across the two lines leading to the terminals of the generator. 
A motor (M) and a number of lamps (L) are shown connected 
in parallel to the terminals of the generator (G), Fig. 213. 
With a change in the number of lamps or motors connected, 
there will be a change in the value of the total resistance 
between the two leads (Li) and (L2) and, as a result, a change 
in the value of the current output of the machine. Thus, for 

257 




258 



PEACTICAL APPLIED ELECTRICITY 



example, if the generator is connected to ten incandescent! 
lamps each having a hot resistance of 220 ohms, there will^ 
be a total resistance of (220-^10) or 22 ohms between the 
leads, and if the voltage of the machine is 110 volts, there 
will be a current of 5 amperes supplied by the machine. Now 
if 5 of the lamps are disconnected, the resistance between the 
leads will be increased as the number of paths in parallel has 
been decreased, and it will be equal to (220 -^ 5) or 44 ohms. 
The current supplied by the generator in this case will be 
2.5 amperes, the voltage remaining constant. When more 
than ten lamps are connected, the combined resistance will 
be less than the resistance of the ten and, as a result, the 
current supplied by the generator will increase, which results 
in an increase in the power output, it being equal to (E X I). 

292. Constant-Current 



Arc Lainps 




Fig. 214 



Distribution. — In the con- 
stant-current, or series 
system of distribution, the 
various devices that are 
being supplied with ener- 
gy are connected in series 
to the terminals of the 
generator. A number of 
arc lamps and a motor 
(M) are shown connected 
in series in Fig. 214, and the combination connected to the 
generator (G). If there is a change in the number of lamps 
or motors connected in such a circuit there will be a change 
in the value of the resistance of the circuit, and since the 
construction of the generator (G) is such that it supplies a 
constant current, there must be a change in the terminal volt- 
age of the machine with a change in load. Thus, if there are 
ten arc lamps connected in series and there is a drop of 90 
volts over each lamp, the terminal voltage of the machine 
must be (10X90) or 900 volts, neglecting the drop in the line 
wires. If five of these lamps be disconnected from the line, 
the circuit still being closed, the voltage required at the ter- 
minals of the machine will be (5X90) or 450 volts. The cur- 
rent in the circuit is the same in each case. The output of 
the machine in the second case then would be only one-half 
what it was when the ten lamps were in operation as the 



DISTEIBUTION AND OPEEATION 



259 



Distribution. — The series- 
combination of the series 
of similar lamps may be 




Fig. 215 



voltage has been reduced one-half, the current remaining 
constant. 

293. Series-Parallel System of 
parallel system of distribution is a 
and parallel systems. A number 
connected in series and the 
combination then connected 
to the supply leads, as 
shown in Fig. 215. 

The same current exists 
in each lamp of a given set 
and the drop in potential 
over the various lamps that 
may be connected in series 
will be proportional to their 

respective resistances. This system is usually used where 
it is desired to operate a number of lamps or motors from a 
line whose voltage is several times that required to operate 
a single lamp or motor. A good example of such an arrange- 
ment is the wiring in a street car where five similar 110-volt 

lamps are connected in se- 
ries and the group then con- 
nected to the 550-volt source 
of supply, as shown in Fig. 
215. If one of the lamps in 
any group should burn out 
or for some reason be re- 
moved from its socket, thus 
opening up the circuit, the 
remaining lamps of that 
group would also be ex- 
tinguished. 

294. Edison Three -Wire 
System of Distribution. — 
Two 110-volt generators, (G^) and (G^), are shown connected in 
series and supplying current to a number of groups of incan- 
descent lamps, Fig. 216. Each group of lamps consists of two 110- 
volt lamps connected in series. If the two lamps of any group 
have the same resistance there will be the same drop in 
potential over each, since they both carry the same current, 
the drop being equal to the product of the resistance between 




Fig. 216 



260 



PEACTICAL APPLIED ELECTEICITY 



1 




the points considered and the current in the resistance. All 
of the points numbered (1), (2), (3), etc., will be at the 
same potential, since the drop over each of the upper 
lamps is the same and equal to the drop over each of the 
lower lamps. If these various points be connected by a con- 
ductor there will be no change in the division of the current 

through the various 
branches or lamps. The 
conductor connecting the 
points (1), (2), and (3) 
may be extended back and 
connected between the two 
generators, as shown in 
Fig. 217, provided the ter- 
minal voltage of the gen- 
erators is the same, the 
point (A) between them 
will be at the same poten- 
Fig. 217 tial as the points (1), (2), 

and (3), and connecting all of these various points together 
by the lead (N), which is called the neutral wire, will pro- 
duce no change in the division of the line current through the 
various branches of the load. Such an arrangement as that 
shown in Fig. 217, is called the Edison three-wire system of 
distribution. 

There will be no current in the neutral wire when the 
number of lamps in the upper set is the same as the number 
of lamps in the lower set and the current in the leads (Lj) 
and (L2) will be the same in value and in the direction indi- 
cated by the arrows in the figure. If the number of lamps 
in the lower group be less than the number in the upper 
group, there will, as a result, be a current in the neutral lead 
and the direction of this current will be toward the gen- 
erators, or toward the left, when the polarity of the machines 
is the same as that shown in the figure. When the number 
of lamps in the upper group is less than the number in the 
lower group, the current in the neutral lead will be from the 
generators or toward the right when the polarity of the 
machines is the same as that indicated in the figure. The 
current in the neutral lead will always be equal to the dif- 
ference in the value of the current in the two outside leads 



d 



DISTEIBUTION AND OPERATION 261 

(Li) and (L2) and its direction will be just opposite to that 
of the current in the outside lead that is greatest in value. 
When there is no current in the neutral lead, the load is said 
to be balanced and when there is a current in the neutral 
lead it is said to be unbalanced. 

The neutral lead in no case will carry a current greater 
in value than either of the outside leads and, as a result, it 
need not be greater in cross-sectional area than the outside 
leads. Each outside wire in the Edison three-wire system 
need be only one-fourth as large in cross-sectional area to 
supply the same number of lamps with the same per cent 
voltage drop as would be required in the simple 110-volt 
system. Only one-fourth as much copper would be required 
in the three-wire system as would be required in the 110-volt 
system if no neutral lead was used, or each outside lead 
would represent one-eighth 

of the total copper re- ^ 75 L 

quired in the 110-volt sys- ~"^~ 
tem. Since the neutral ' 

lead is usually made equal 
in area to the outside •^•^^^^-^ ^ 25 

--N- 




leads, there will be three 
leads, each representing in 
weight one-eighth of the 
copper required for the "^'.'--'^-y^ ^^ 

110-volt system, or the to- >1^ /- "^ " ^^ 

tal copper required for the " p. 218 

three-wire system is three- 
eighths of that required for the 110-volt system. 

If no neutral lead were used, it would be impossible to 
operate one lamp alone since there are two in series; but 
with the neutral lead, the number of lamps that may be turned 
on or off in either the upper or the lower groups is entirely 
independent of the number of lamps turned on in the other 
group. 

295. Drop in Potential in the Neutral Wire.— If the current 
taken by the upper group of lamps in Fig. 217 be 75 amperes 
and the current taken by the lower group be 50 amperes, 
there will be a current of 25 amperes in the neutral lead and 
the direction of these currents will be that shown by the 
arrows in Fig. 218. Since the current in the neutral lead is 



262 PEACTICAL APPLIED ELECTRICITY 

toward the generators, the end connected to the load mu^t be 
at a higher potential than the end connected to the , gen- 
erators. The load end of the upper lead (Lj) is at a lower 
potential than the generator end and the load end of the 
lower lea'd (L2) is at a higher potential than the generator 
end. The drop in potential in these various leads is repre- 
sented by the dotted lines in Fig. 218, the potential falling 
off along each lead in the direction of the current. Assuming 
the voltage of each of the generators is 112 volts and that 
this voltage remains constant regardless of the load, then the 
voltage over the upper group of lamps will be equal to 112 
minus the algebraic sum of the drops in the upper lead (Lj) 
and the neutral; while the voltage over the lower group of 
lamps will be equal to 112 minus the algebraic sum of the 
drop in the lower lead (L2) and the drop in the neutral. The 
drop in the neutral causes a decrease in the voltage over the 
upper group of lamps and it tends to increase the voltage over 
the lower group of lamps. The voltage over the smaller load 
will be greater than the terminal voltage of the machine con- 
nected to that load when the current in the neutral is greater 
than the current in the ouside lead connected to the smaller 
load, if the neutral and outside leads have the same resistance. 
If a three-wire system be unbalanced and the fuse in the 
neutral lead should blow or the neutral should be opened, the 
outside leads remaining connected to the generators, there will 
be a redistribution of the total voltage, between the two leads 
(Li) and (L2), over the upper and lower groups of lamps. The 
drops over the two groups would bear the same relation to 
each other as exists between the resistances of the two 
groups, that is, the group containing the smaller number of 
lamps or having the larger resistance will have the larger 
drop over it. For example, if there be ten 220-ohm lamps 
connected in parallel and forming one group and only one 220- 
ohm lamp in the other group, the resistances of the two 
groups would be in the ratio of 22 to 220. This would result 
in the voltage over the single lamp being equal to 22o^_j2 of 
the total voltage between the outside leads when the neutral 
lead was open. If the total voltage between the outside leads 
is, say 220 volts, then the voltage over the single lamp will be 
200 volts. This voltage is in excess of the value the lamp will 
stand and, as a result, the lamp filament will be destroyed. 



DISTEIBUTION AND OPEEATION 



263 



296. Three-Wire Generators. — In the operation of the 
Edison three-wire system as described in sections (294) and 
(295), two generators are required, giving rise to an addi- 
tional expense for machines as compared to the system using 
a single generator; and on this account a special type of 
machine has been devised which is known as a three-wire 
generator. 

The total voltage of any direct- 
current generator can be divided 
into two parts by placing a brush- 
midway between the negative and 
the positive brushes of the ma- 
chine. The coils short-circuited 
by this additional brush would be^ 
in a strong magnetic field in the 
ordinary machine and, as a result, 
there would be considerable 
trouble encountered in properly 

commutating the current. This additional brush, however, can 
be connected to a coil that is located in a weak magnetic field 
and the commutation greatly improved by the arrangement 
shown in Fig. 219, which consists of a four-pole field-magnet 
frame wound for a bipolar machine, there being two adjacent 
north poles and two adjacent south poles. The brushes (Bj) 




Fig. 219 




Fig. 220 



and (B2) represent the main brushes of the machine and the 
brush (B3) is the one to which the neutral lead is connected. 
The brush (B3) is negative with respect to the brush (Bj) 
and positive with respect to the brush (B2). The magnetic 



264 



PEACTICAL APPLIED ELECTRICITY 



flux from the two north poles or into the two south poles is 
not equal on account of armature reaction which crowds the 
flux into the forward pole. This results in the voltage between 
the neutral brush and one main brush being greater in value 
than the voltage between the neutral brush and the other 
main brush. 

Another form of three-wire generator is shown diagram- 
matically in Fig. 220. The armature winding is divided into 




Pig. 221 



the same number of parts as there are magnetic poles on the 
machine. The alternate division points of the armature 
winding are connected to one slip ring and the remaining 
division points to a second slip ring. A coil of low resistance 
and high inductance, called a reactor, is connected to two 
brushes that make continuous contact with the two slip rings, 
and a tap, which is taken off from the center of this coil, 
forms the neutral connection. 



mJSTKIBUTION AND OPERATION 265 

The number of slip rings may be reduced to one by placing 
the reactor inside of the armature and allowing it to revolve 
with the armature, the middle point being connected to the 
slip ring. Three-wire generators may be flat or over-com- 
pounded just as an ordinary generator to compensate for 
armature and line drops. A machine manufactured by the 
General Electric Company and provided with two slip rings 
is shown in Fig. 221. 

297. Dynamotors. — The dynamotor is a machine having 
one magnetic field and two armature windings. It is a com- 
bination of generator and motor. Each of the armature wind- 
ings is usually supplied with a separate commutator, the 
electrical connections of the two windings being independent. 
Either of these windings may be used as a generator or a 
motor. The relation of the terminal voltage of the generator 
side and the impressed voltage on the motor side will depend 
upon the relation between the number of turns in the two 
armature windings. If the number of inductors in the winding 
of the generator armature winding is one-fourth the number 
of inductors in the motor armature winding, then the voltage 
of the generator will be practically one-fourth the voltage 
impressed upon the motor armature. The current output of 
the generator, however, will be practically four times the cur- 
rent taken by the motor. This relation of current and volt- 
age in the generator and the motor armatures results in there 
being practically the same number of ampere-turns in each 
armature winding when the machine is in operation, and since 
the current in the generator armature winding will be in the 
opposite direction around the armature core to what it is in 
the motor armature winding, there will be practically no 
armature reaction, the two magnetizing effects neutralizing 
each other. 

The dynamotor in the direct-current circuit corresponds to 
the transformer in the alternating-current circuit, however, it 
is not nearly so eflicient as the transformer. 

A dynamotor manufactured by the Crocker Wheeler Com- 
pany is shown in Fig. 222. 

298. Dynamotor as an Equalizer. — The dynamotor is often 
used to equalize the difference in potential between the out- 
side leads in a three-wire system when one side of the system 
Is carrying a larger load than the other. When the dyna- 



266 PEACTICAL APPLIED ELECTKICITY 

mometer is thus used, it is called an equalizer; the wind- 
ings on the armatures are the same and the machine can 
be connected to the line as shown in Fig. 223. When the 
two sides of the system are balanced there will be no 
current in the neutral lead (N) and a small current will 
pass through the two armature windings of the dyna- 
motor in series, both armatures acting as motors. If one side 




Fig. 222 

of the system is carrying a larger load than the other, there 
will be a greater drop in the leads connected to it and, as a 
result, a lower voltage will exist over the larger load than 
exists over the smaller load. The armature winding of the 
balancer connected to the higher voltage will act as a motor 
and drive the other armature winding which will act as a 
generator, and the pressure of this generator will tend to 
raise the voltage of the more heavily loaded side. When one 
of the windings changes from a motor to a generator, the 
current in it reverses in direction. The direction of the 
currents in an unbalanced three-wire system that is being 
supplied with energy from a main generator (G) is shown in 
Fig. 223. The upper commutator of the balancer is connected 
to the generator winding of the dynamotor and is supplying 
current to the upper or larger load, and the lower commutator 
is connected to the motor winding of the dynamotor and is 
taking current from the lightly loaded side. 



DISTRIBUTION AND OPERATION 



367 




DM Ls=_nI_1 



C£ 



Fig. 223 



299. Motor-Generators, or Balancers. — A motor-generator in 
its simplest form consists of a motor mechanically connected 
to a generator. The number of generators connected to any 
• motor is not limited to a single machine, but it may be any 
number. Thus, a motor may 
be electrically connected to 
a source of energy and be 
operated as any ordinary 
motor, its output being con- 
sumed in driving a number 
of generators of the same or 
unequal voltages. These va- 
rious generators can in turn 
be connected and supply en- 
ergy to a multi-voltage system, as shown in Fig. 224. 

A motor-generator is quite often used in connection with a 
three-wire system, the motor and the generator being similar 
machines and rigidly connected together. The combination, 
when so used, is called a balancer and its operation is prac- 
tically the same as the dynamotor previously described. The 

connection of a balancer composed 
of two simple shunt generators to a 
three-wire system is shown in Fig. 
225. The two machines composing 
the balancer have their field con- 
nections interchanged, that is, the 
field of one machine is connected 
to the terminals of the other. With 
this arrangement of connections, 
the voltage regulation is greatly im- 
proved, and is further improved by 
compounding the two machines 
forming the balancer and connect- 
ing them as shown in Fig. 226. 

300. Boosters. — When electrical 
energy is being distributed from a 
central station over long leads, there is a drop in volt- 
age due to the resistance of the, leads and, as a result, the 
voltage at the receiving end of the line is less than it is at the 
transmitting end. This loss in voltage can be compensated 
for by connecting in series with the line a machine called a 




268 



PEACTICAL APPLIED ELECTRiClTy 



booster, whose action in the circuit is to produce an elec- 
trical pressure which acts in series with the main gen- 
erator pressure. When the electrical pressure of the 
booster acts in opposition to the voltage of the main 
generator, it is called a negative booster. The booster 
may be driven by a motor connected to the same line 



the booster is connected to, or 




Fig. 225 



to any other line, or it 
may be engine -driven. 
There are a number of dif- 
ferent forms of boosters 
but only two will be men- 
tioned hare, the series and 
the shunt. The connection 
of a series booster in a line 
is shown in Fig. 227. The 
booster (B) is driven by a 
compound motor (M) con- 
nected as shown in the figure. The booster itself is nothing 
more than a series generator in which the iron of the mag- 
netic circuit will not be worked above the knee of the curve 
when the maximum current which the feeder is to carry 
passes through the series winding of the booster. When the 
magnetic circuit of the machine is worked at such a flux 
density, the relation between the terminal voltage of the 
machine and the load cur- 
rent is practically a 
straight line. Now by prop- 
erly winding the machine, 
its terminal voltage may 
be made to increase or de- 
crease with a change in 
current in the feeder just 
a sufficient amount to ex- 
actly compensate for the 
loss in voltage due to copper drop. 

In charging a storage battery, the voltage of the line from 
which current is to be taken in charging the battery may in 
some cases be less than is required to give the battery a full 
charge. In such a case a small shunt-wound generator (B) 
may be connected in series with the battery, as shown in Fig. 
228. The voltage of this generator will act in series with that 




Fig. 226 



DISTEIBUTION AND OPERATION 



269 




Fig. 227 



of the line giving a resultant voltage sufficient to charge the 
battery. A shunt generator when so used is called a shunt 
booster and it may be either motor- or engine-driven. 

301. Operation of Generators and Motors for Combined 
Output. — The load generating stations are called upon to carry 
is, as a rule, not constant 

in value throughout the 
twenty-four hours of the 
day. If one large generat- 
ing unit were installed in 
a station supplying a vary- 
ing load, the efficiency of 
the plant would vary be- 
tween very wide limits on 
account of the efficiency of 

a generator varying with the load it is carrying, A generator 
is usually designed so that it will have the maximum efficiency 
at its rated full load, it decreasing in value with either an in- 
crease or decrease in load. Now in order that the generating 
equipment in a central station be operated at its maximum 
efficiency, it should at all times be carrying a load something 

near its full capacity. In 
^ — ^ — ^ L, modern central stations 

this is accomplished by 
using a number of genera- 
tors, instead of a single 
machine, their combined 
capacity being sufficient to 
carry the full load of the 
station and so arranged 
that they may all be con- 
nected to the load at the same time. The number of genera- 
tors in operation may be changed as the load on the station 
changes and in this way each machine will operate on a load 
corresponding to, or near, its maximum efficiency. 

For a similar reason to that mentioned above, motors are 
connected so that their outputs may be added, the number of 
motors in operation depending upon the load. 

302. Shunt Generators Connected for Combined Output. — • 
Two simple constant-voltage shunt generators (Gi) and (G2) 
are shown connected in parallel, in Fig. 229, to two heavy 




Fig. 228 



270 



PRACTICAL APPLIED ELECTRICITY 






leads (Li) and (L2) called bus-bars. The regulating rheostats 
(Ri) and (R2) in the field circuits should be adjusted so that 
the total load connected to the bus-bars is properly divided 
between the two generators. If the voltage regulation of the 

two machines is not maintained, no 
serious damage will result except 
one machine will not carry its por- 
tion of the load, the machine of 
higher voltage carrying the greater 
portion of the load. The voltage of 
one machine may drop to such a 
value that it will change to a motor 
but still no serious damage will re- 
Pig 229 suit as the shunt motor rotates in 
the same direction as a shunt gen- 
erator, the connections of the field windings and armature 
leads remaining unchanged. 

Two or more shunt generators may be operated in series, 
as shown in Fig. 224, and supply energy to a multi-voltage 
system or they may supply energy to a single voltage system, 
they being connected in series so as to increase the total 
voltage between the leads. 

303. Series Generators Con- 
nected for Combined Output. — Two 
series generators (Gi) and (G2) 
are shown connected in parallel to 
the bus-bars (L^) and (L2), Fig. 
230. The two machines will oper- 
ate satisfactorily in parallel so 
long as their voltages remain the 
same. If, however, the voltage of 
one machine falls a small amount 

due to any cause, such as a decrease in speed of its prime 
mover, there will be a decrease in current supplied by it and, 
as a result, a decrease in its field excitation, which results in 
a further decrease in its voltage. This unbalanced condition 
continues to grow until the lower voltage machine is con- 
verted into a motor, and since the direction of rotation of a 
series generator is opposite to what it is when used as a 
motor, the results may be very serious to one or both ma- 
chines. 




Fig. 230 



i 



DISTRIBUTION AND OPERATION 



271 




Fig. 231 



The above difficulty in operating two series-wound gen- 
erators in parallel can be overcome to a certain extent by 
allowing the armature current of one machine to pass through 
the field of the other. With this arrangement, a decrease in 
armature current of one machine causes a decrease in voltage 
of the other machine and, as a result, 
the first machine must carry its proper 
share of the load. 

The current in the various field 
windings may be made independent of 
the voltage of the different machines 
by connecting the junctions of the 
series windings and brushes by a low 
resistance lead, called an equalizer, as 
shown in Fig. 231. The polarity of 
the points connected by the equalizer 
should all be the same. When this connection is made, the 
current in the various series windings is practically the same 
provided they have the same resistance. It is impractical to 
operate series generators in parallel and, as a result, it is 
never done. 

Series generators are operated in series in practice in 
direct-current, high voltage power transmission. The gen- 
erators and their fields are 
all connected in series. 
Such systems are used in 
Europe and in the major- 
ity of cases they are con- 
stant-current systems, the 
current being maintained 
constant in value by spe- 
cial regulating devices 
which change both the 
speed of the prime movers 
and the position of the 
brushes. 
304. Compound Generators in Paraiiel. — Compound gen- 
erators may be operated in parallel very satisfactorily, the 
connections being made as shown in Fig. 232. It is not 
necessary that the capacity of the machines connected in 
parallel be the same but they must have the same terminal 




5hunt I 



Shunt 
Fig. 232 



272 PRACTICAL APPLIED ELECTRICITY 

voltage at all loads and the resistance of their series fields 
must be to each other inversely as their capacities for the 
same degree of compounding in order that the total load be 
divided in proportion to their respective capacities as the load 
changes in value. When it is desired to operate two machines 
having different degrees of compounding, the series field of 
the machine of higher compounding should be shunted with 
a resistance, or sufficient turns in the series windings discon- 
nected, so that its compounding will correspond to that of 
the machine with which it is to operate. A resistance may 
be connected in parallel with the series winding of one of the 
machines, if the currents in the series windings are not in- 
versely as the capacities of the two machines. 

305. Operation of Shunt Motors in Series and Parallel. — 
Any number of shunt motors will operate satisfactorily in 
parallel across a constant pressure, and each motor may be 
connected to a separate load or to the same load. 

When a number of shunt motors are operated in series 
across a constant-pressure line, they must be rigidly connected 
together. If they were not rigidly connected together and the 
load was removed from one of the motors, it would race and 
rob the remaining motors of their proper share of the total 
voltage. It is not practical for this reason to operate shunt 
motors in series unless they be rigidly connected together. 

306. Operation of Series Motors in Series and Parallel. — 
Any number of series motors may be operated in series on 
constant-current circuits and their operation is independent 
of the load any of the motors may be carrying. Any motor 
may be overloaded until it stops without interfering with the 
other motors connected to the same line, since the current in 
the circuit remains constant in value at all times. Series 
motors will operate satisfactorily in series on constant-voltage 
circuits provided they are rigidly connected together. An 
example is to be found in starting a street car when the 
motors are connected in series groups of two each. The 
speed of each motor is the same in this case, provided none 
of the wheels slip, and the voltage over each will be prac- 
tically the same. If, however, the wheels to which one of the 
motors is geared slip, this motor will speed up and rob the 
other motor of its proper share of the line voltage and, as a 



DISTEIBUTIOlSr AND OPEEATION 373 

result, the motor having the lower voltage impressed upon it 
will have its starting torque lowered. 

Any number of series motors may be operated in parallel 
from a constant-voltage line provided their loads are not dis- 
connected, which would result in the motor racing and no 
doubt destroying itself. 

307. Operation of Compound Motors. — Compound motors 
are operated from constant-voltage lines in every case and 
each has its own load. The speed regulation of the compound- 
wound motor, when the series- and shunt-field windings are 
differentially connected, is better than any other type of 
direct-current motor. 

308. Switchboard. — A switchboard is a board, of insulating 
material usually, upon which the indicating instruments and 
the switches used in connecting various electrical circuits are 
located. It is customary to divide a switchboard up into 
sections called panels. If a switchboard be used in connect- 
ing a number of machines to a certain load, it will be divided 
into what are called generator panels and feeder panels. All 
of the switches and instruments associated with each genera- 
tor being mounted on the generator panels, and the instru- 
ments and switches associated with the feeder circuits being 
mounted on the feeder panels. The equipment most com- 
monly found on a direct-current switchboard consists of volt- 
meters, ammeters, wattmeters, ground detectors, circuit 
breakers, fuses, rheostats, and switches. All of the above 
equipment has been described with the exception of circuit 
breakers, rheostats, ground detectors, and switches. These 
devices will be discussed in the following sections. 

309. Circuit Breakers. — A circuit breaker is a switch that 
may be closed against the action of gravity or a spring and 
held in the closed position by means of a suitable latch, which 
in turn is controlled by one or more solenoids. A solenoid 
may be connected so that its winding will carry all or a 
definite part of the total current the contacts of the circuit 
breaker carry and its armature may be so adjusted that it 
will be drawn up and trip the latch when the current exceeds 
a. certain value. Such a circuit breaker is called an over- 
load circuit breaker. 

A solenoid may be so constructed that it will trip the latch 



274 



PRACTICAL APPLIED ELECTRICITY 



of the circuit breaker when the current in the circuit is 
reversed. Such a circuit breaker is called a reverse-current 
circuit breaker. 

A circuit breaker in which the latch is tripped when the 
current falls below a certain value is called an under-load 
circuit breaker. 

A fourth form of circuit breaker is one having the solenoid 
controlling the latch connected directly across the line and so 
arranged that* the breaker is opened automatically when the 
voltage drops below a certain value. This form of circuit 
breaker is called a no-voltage circuit breaker. Various com- 
binations of the above forms of circuit breakers may be made 
which will meet practically all requirements. An over-load 
and no-voltage circuit breaker combined is shown in Fig. 233. 

310. Rheostat. — A rheostat is a 
resistance whose value may be 
varied. There are numerous forms 
of rheostats, their construction in a 
measure being determined by the 
particular use to which they are to 
be placed. A form of rheostat used 
in regulating the shunt-field current 
of generators is shown in Fig. 234. 
This consists of a number of small 
coils of wire connected in series 
and their junctions joined to a 
number of contact buttons over 
which a -metal arm passes. One 
terminal of the rheostat is the arm 
that moves over the contact but- 
tons and the other terminal is one 
of the end coils. The variation in 
current that may be produced by such a rheostat will depend 
upon the relation between the resistance of the circuit in 
which the rheostat is connected and its own total resistance. 
Such rheostats, as a rule, have rather a small current- 
tarrying capacity. 

A rheostat composed of a number of carbon plates mounted 
side by side is shown in Fig. 235. The resistance of this 
rheostat is changed by varying the pressure between the 
various plates of carbon between the end plates, by means of 




Fig. 233 



DISTRIBUTION AND OPERATION 



275 



a small handle shown in the figure. The resistance of this 
rheostat is, as a rule, rather low, but its current-carrying 
capacity is quite large. It 
operates very satisfactorily 
in low voltage circuits and 
the variations in its resist- 
ance can be made very 
gradual. 

Rheostats are usually 
controlled from the switch- 
boards when used in ad- 
justing the field current of 
machines and the control 
is accomplished by a small 
handle on the face of the 
board, the rheostat being 
usually mounted back of 
the board. 

311. Ground Detectors. — A ground detector is an instru- 
ment used in measuring the insulation resistance between any 
line connected to the switchboard and ground. It is really a 
special form of series voltmeter marked in some cases to read 
directly in ohms. 

312. Switches. — Switches are devices that are connected in 




Fig. 234 




Fig. 235 



a circuit to facilitate its being closed or opened. They may be 
single-pole, double-pole, etc., depending upon the number of 
circuits that are interrupted when the switch is opened. A 
double-pole double-throw switch is shown in Fig. 236. This 
switch is so constructed that the circuits are connected to it 
back of the board upon which it is mounted. The size of the 



276 PRACTICAL APPLIED ELECTRICITY 

jaws and blades of a switch will depend upon the value of the 
current the switch is designed to carry, and the distance 
between the various parts that are connected to the different 
leads will depend upon the voltage. 

313. Instructions for Starting a Generator or Motor. — In 
starting a machine make sure the commutator is perfectly 
clean. Examine the brushes carefully to see that they are all 
making good contact with the commutator and that they are 
in their proper position, which should be indicated by a mark 
on the rocker arm supporting them. If there happens to be 
no such mark, their position must be adjusted after the 
machine is in operation, it being indicated by a maximum 
voltage in the case of a generator and a minimum speed in 




Fig. 236 

the case of a motor. They are, however, usually advanced 
a little beyond this position in a generator and back of it in 
a motor in order to reduce the tendency for sparking. Exam- 
ine all connections and see that all screws and bolts are tight. 
Fill the oil cups and see that they are supplying oil to the 
parts they are supposed to lubricate. 

If the machine is being started for the first time, make sure 
that it turns over freely and that the armature is properly 
balanced and that it is centrally located in the magnetic field. 
After it has been thus inspected, the machine should be 
started up and its speed increased gradually when possible, 
with the switches all left open in the case of a generator. 
If the machine is being started for the first time, it should 
be run for a number of hours without load and the load then 



I 



DISTEIBUTION AND OPERATION" 



277 




Fig. 237 



278 PRACTICAL APPLIED ELECTRICITY 

gradually applied, the attendant being in readiness at all 
times to disconnect or stop it if anything should go wrong. 

314. Starting and Stopping Compound Generators tiiat are 
Operating in Parallel.-— A diagram of the wiring of a switch- 
board used in connecting two generators in parallel and to a 
common load or in connecting either generator to a separate 
load is shown in Fig. 237. The left-hand panel is the gen- 
erator panel for, say generator (Gi); the middle panel is the 
generator panel for generator (G2) ; and the right-hand panel 
is the feeder or load panel. On each generator panel there is 
mounted a wattmeter (W), a circuit breaker (CB), a main 
generator switch, an equalizer switch, a field-regulating rheo- 
stat (shown fn the figure by the dotted circles), a voltmeter, 
and an ammeter. The ammeter shunt is shown connected in 
series with the lead from the generator and it should always 
be connected in the lead that goes to the terminal of the 
generator opposite the one to which the equalizer lead is 
connected. If it were connected in the other lead it would not 
necessarily indicate the current that really exists in the arma- 
ture of the generator unless there was no current in the 
equalizer. 

The following apparatus is mounted on the feeder panel 
shown in Fig. 237. Two lines (Li) and (L2) are connected 
to the central points of the double-pole double-throw switches 
(S3) and (S5), one lead of each line passing through the cur- 
rent coils of the two wattmeters (W3) and (W4). The outside 
points of the switches (S3) and (S5) are connected to the 
bus-bars of the two generators (G^) and (G2). The upper and 
the lower contacts of the switch (S4) are connected to the bus- 
bars of the two generators and when this switch is closed the 
two machines will be connected in parallel, provided the 
terminals of the switch (S4) that are connected when the 
switch is closed are of the same polarity. A voltmeter (Vc) 
is mounted on this panel and its terminals are connected to 
the central points of the small double-pole double-throw switch 
(Sg), which has its upper and lower contacts connected to the 
terminals of the two generators (Gi) and (G2). 

Assuming now one machine is in operation and connected 
to one or both of the loads and it is desired to connect the 
idle machine in parallel with the one already in operation. 
The two loads can be connected to one machine by closing the 



I 



DISTRIBUTION AND OPERATION 279 

switches (S3), (S4), (S5), and the proper generator switch. 
Bring the generator up to speed, close the equalizer switches 
(Se) and (S7), and adjust its shunt-field current to such a 
value that its terminal voltage is a little in excess of that 
between the bus-bar to which the machine is to be connected. 
This can be determined by noting the indications of the two 
voltmeters (Vi) and (V2), respectively, or the voltmeter (Vc) 
may be thrown from one circuit to the other, by operating the 
switch (Sg) and its indication noted when it is connected to 
the two circuits. Using a single voltmeter eliminates the 
possibility of an error due to the two separate voltmeters 
not indicating the same, even though they be connected to 
the same pressure. When the voltage of the incoming ma- 
chine has been adjusted to the proper value, the main switch 
may be closed and the field current adjusted to such a value 
that the load is divided between the two machines in propor- 
tion to their capacities. You should always be absolutely 
sure that the points of the last switch you close in connecting 
the machines in parallel is of the proper polarity, positive to 
positive and negative to negative, before the switch is closed. 
If the indications of the voltmeters depend upon the direction 
of the current in their windings, and you are sure there has 
been no change in the connections back of the board or at the 
machines, you can then close the paralleling switch when all 
of the voltmeters read in the proper direction. 

When it is desired to disconnect a machine from the line, its 
voltage is lowered by reducing its shunt-field current and, as 
a result, it fails to carry its proper share of the load. The 
main generator switch should be opened when the current 
output of the generator has decreased to almost zero value. 
The equalizer switch may then be opened and the machine 
shut down. 



CHAPTER XIV 

DISEASES OF DIRECT-CURRENT DYNAMOS 

315. Sparking, at the Brushes Due to Fault of the 
Brushes. — 

(1) Brushes not set diametrically opposite. 

(Rla) Should have been properly set at first while 
at rest by counting bars, by measurement, or by use 
of reference marks on the commutator. 

(Rib) Can be done if necessary while running by 
bringing the brushes on one side to the least 
sparking point by moving the rocker arm and then 
adjusting the brushes on the other side to the least 
sparking point by moving the rocker arm and then 
brush holder and then clamping. 

(2) Brushes not set in neutral point. 

(R2a) Move the rocker arm slowly back and foith 
until the sparking stops. See (R58e). 

(3) Brushes not properly trimmed. 

(R3a) Brushes should be always kept properly 
trimmed and set. If sparking begins from this 
cause and dynamo cannot be shut down, bend back 
the brushes and cut off loose and ragged wires if 
metal brushes are used. Retrim as soon as pos- 
sible after run is over. If there are two or more 
brushes in^ each set, they may be changed one at 
a time for new and properly trimmed ones during 
the run on any low voltage machine. See number 
(38). To trim, clean them from oil or dirt with 
benzine, soda, or potash, then file or grind to a 

Note. — The list of dynamo diseases given in this chapter 
was taken from a large chart, suitable for framing, that is 
published by the Guarantee Electric Company, Chicago. 

280 



DISEASES OF DIRECT-CUERENT DYNAMOS 281 

standard jig and reset carefully as at (Rla), (Rib). 
See (R38a) and (R38b). 

(4) Brushes not in line. 

(R4a) Adjust each brush of a given set until they 
are all in line and square with the same commu- 
tator bar, bearing evenly for their entire width, 
unless purposely staggered. See (R13a). 

(5) Brushes not in good contact. 

(R5a) Clean the commutator of all dirt, oil or grit, 

so that brushes touch. 
(R5b) Adjust pressure by tension screws and 
springs until light, firm, yet even contact is made. 
Pressure should be about 1.25 pounds per square 
inch. See number (38). 
316. Sparking at the Brushes Due to Fault of the Com- 
mutator or Magnetic Field. — 

(6) Commutator rough, worn in grooves or ridges. 

(7) Commutator not round. 

(R6a and R7a) Grind down the commutator with fine 
sand paper (never emery in any form) laid in stock 
curved to fit the commutator. Polish with a soft, 
clean cloth. 

(R6a and R7b) If too bad to grind down, turn off with 
a special tool and rest while turning slowly in the 
bearings, or remove the armature from bearings 
and turn off with light cuts in lathe. 

Note. — Armature should have from ^q'' to y^'' end 
motion so as to distribute wear evenly and prevent 
wearing in ruts or ridges. Brushes may be shifted 
sideways occasionally to assist in distribution of 
the wear. See number (31). 

(8) One or more high commutator bars. 

(R8a) Set the high bar down carefully with a mal- 
let or block of wood, being careful not to bend, 
bruise or injure the bar and then tighten the 
clamping rings. If this does not remedy the fault, 
file, grind or turn the high bar down to the level of 
the other bars. The high bar may cause the 
brushes to jump or vibrate so as to *'sing." See 
number (38). 

(9) One or more low commutator bars. 



282 PRACTICAL APPLIED ELECTRICITY 

(R9a) Grind the remainder of the commutator down 
to a true surface so as to remove low spots. 

Note. — The insulation between the segments may be 
high, due to its not wearing as fast as the metal of 
the segments. Insulation should be turned down to 
level of segments to remedy this fault. 

(10) Weak magnetic field. 

(A) Broken circuit in field. 

(RlOa) Solder or repair broken connection. Re- 
wind if the brake is inside of the winding. 

(B) Short-circuit of the coils. 

(RlOb) Repair if external and rewind if internal 

(C) Dynamo not properly wound or without proper 
amount of iron. 

(RlOc) No remedy but to rebuild. 
317. Sparking at the Brushes Caused by an Excessive Cur 
rent in the Armature Due to an Overload. — 

(11) Generator. (A) Too many lamps on the circuit 

(constant-potential system). 

(B) Ground and leak from short-circuit 
on the line. 

(C) Dead short-circuit on the line. 
Motor. (D) Excessive voltage on a constant- 
potential circuit. 

(E) Excessive amperage on a constant 
current circuit. 

(F) Friction. See section (321). 

(G) Too great a load on the pulley 
See section (321). Il 

(Rlla) Reduce the number of lamps and thus di- ' 
minish the current called for. 

(Rllb) Test out, locate the ground and repair. 

(Rile) Dead short-circuit will or should blow th( 
safety fuse. Shut down the dynamo, locate and 
repair the fault. Put in new fuse before starting 
again. Fuse should not be inserted until the faull 
is corrected, as it will blow again on starting ui 
the machine. 

(Rlld) Use the proper value of current for a motoi 
and no other. 



L. 



DISEASES OF DIEECT-CUERENT DYNAMOS 283 

(Rile) Make sure you^ have the proper rheostat or 

controlling switch. 
(Rllf) Reduce the load to the proper amount for 

rating of the motors. 
(Rllg) Remedy any cause of trouble from undue 
friction. See section (321). 
318. Sparking at the Brushes Due to Fault of the Arma- 
ture. — 

(12) Short-circuited coil in the armature. 

(R12a) Look for copper dust, solder, or other cause 
ior metallic contact between commutator bars and 
remove. 

(R12b) See that the clamping rings are properly 
insulated from commutator bars, and from car- 
bonized oil and copper dust or dirt which may form 
a short-circuit. 

(R12c) Test for internal short-circuit or cross-con- 
nection; if found, reinsulate the conductor, change 
the connection, or rewind armature to correct. 

(R12d) Examine the insulation of the brush holders 
for the fault. Dirt, oil, or copper dust may form a 
short-circuit from brush holder to rocker arm, and 
thus short-circuit the machine. 

(13) Broken circuit in the armature. 

(R13a) Bridge the break temporarily by staggering 
the brushes till the run is finished, then test out 
and repair the fault. This is only a temporary 
make-shift to try to stop the bar sparking during 
a run when dynamo cannot be shut down. 

(R13b) If the dynamo can be shut down, look for 
broken or loose connection to the bar and repair. 

(R13c) If the coil is broken inside, rewinding is the 
only sure remedy. The break may be bridged tem- 
porarily by hammering the disconnected bar until 
it makes contact across the mica to the next bar 
of the commutator. This remedy is of doubtful 
value if done. The bars must be repaired and 
insulation replaced again after fault is corrected. 

(R13d) Solder the commutator lugs together or 
bridge across them with piece of heavy wire and 
thus cut out the broken coil. Be careful not to 



284 PRACTICAL APPLIED ELECTRICITY 

short-circuit a good coil in soldering, and thus 
cause sparking from a short-circuited coil, as in 
number (12). 

(14) Cross-connection in the armature. 

(R14a) Cross-connections may have the same effect 
as . a short-circuit and they are to be treated as 
such. See number (12). Each coil should ^tow a 
complete circuit with no connection to any other 
coils. 

319. Heating of the Armature. — 

(15) Overloaded or not centrally located between the 
poles. 

(16) Short-circuit. See numbers (11), (12), (13), and 
(14). 

(17) Broken circuit. 

(18) Cross-connection. 

(19) Moisture in the armature coils. 

(R19a) Dry out the coils by slow heat, which may 
be done by sending a current through the armature 
regulated not to exceed the proper value. If not so 
bad as to cause a short-circuit, cross-connection, or 
too much heat, the moisture may be dried out by 
the heat of its own current while running. 

(20) Eddy currents in the armature core. 

(R20a) The iron of the core may be hotter than the 
coils after a short run due to a faulty armature 
core, which should be finely laminated and the 
laminae insulated. No remedy but to rebuild. 

(21) Friction. 

(R21a) Hot bars and journals may affect the tem- 
perature of the armature. See section (321). 

320. Heating of the Field Coils. — 

(22) An excessive current in the field circuit. 

(R22a) Shunt machine. Decrease the voltage at the 
terminals by reducing the speed, or increasing the 
resistance of the field coils by winding on more 
wire, or rewind them with finer wire, or put a 
resistance in series with 'the field. 

(R22b) Series machine. Shunt a portion of the 
current, or otherwise decrease the current in the 
field windings, or take off one or more layers of 



DISEAfeJ^^S OF DIEECT-CUEEENT DYNAMOS 285 

wire, or rewind the fields with a coarser wire. 
Note. — An excessive current nay be due to a short- 
circuit or from moisture in the coils acting as a 
short-circuit. See number (24). 

(23) Eddy currents in the pole pieces. 

(R23a) The pole pieces may be hotter than the field 
coils after a short run due to faulty construction 
or to a fluctuating current in the latter; regulate 
and steady the current. 

(24) Moisture in the field coils. 

(R24a) The coils show a resistance lower than nor- 
mal, which may be caused by a short-circuit or 
contact with the iron of the dynamo. Dry out the 
coils as in number (19). See note under (R22b). 
321. Heating of the Bearings. — 

(25) Not enough or poor quality of oil. 

(R25a) Supply plenty of good clean oil and see that 
it feeds properly. Oil should be best quality min- 
eral oil, filtered clean and free from grit. 

Note. — Be careful not to flood the bearings so as to 
force oil upon the commutator, or into the insula- 
tion of the brush holders, as it will gradually char 
and gather copper dust and form a short-circuit. 
See (R12b) and (R12d). 

(R25b) Vaseline, cylinder oil, or other heavy lubri- 
cant may be used if ordinary oil fails to remedy 
the hot box. Use till run is over, then clean up 
and adjust the bearings. 

(26) Dirt, grit, or other foreign matter in the bearings. 
(R26a) Wash out the grit by flooding the bearings 

with clean oil until run is over. Be careful, how- 
ever, about flooding the commutator or brush 
holders. See note under (R25a). 

(R26b) Remove the cap and clean the journals and 
bearings, then replace the cap and lubricate well. 

(R26c) After the run is over (or if shut-down is 
made), remove the bearings completely, cool off 
naturally, and polish everything free from grit, and 
set up again. 

(27) Rough journals or bearings. 

(R27a) Smooth and polish in a lathe, removing all 



1 



286 PRACTICAL APPLIED ELECTRICITY 

cuts, burs, scratches, and tool marks, then make 
new bearings (of babbitt or other metal) to prop- 
erly fit. 

(28) Journals too tight for bearings. 
(R28a) Slacken the bolts in the cap, put in packing 

pieces till run is over, then fit to smooth bearing 
and easy rotation by hand (if the machine is 
small). 
(R28b) Turn down smooth and repolish the journal, 
or ream or scrape the bearings until they fit prop- J 
erly. 

(29) Bent or sprung shaft. 
(R29a) Bend or turn the shaft true by careful] 

springing or turning in lathe. 

(30) Bearings out of line. 
(R30a) Loosen the base of the bearings and shift 

them until the armature turns freely by hand with 
belt off and it is at the same time in the center of 
the polar space. Remount, bolt, and dowel-pin 
holes, and fit new dowels to allow new position to 
be kept when bolts are drawn up tight. If shaft 
need be raised or lowered, then pack up or trim 
down the foot of the bearing to allow the proper 
setting. 

(31) End pressure of the pulley hub or shaft collars 
against the bearings. 

(R31a) See that the foundation is level and that the 
armature moves freely with a small amount of end^ 
motion. ^11 

(R31b) If there is no end motion, then turn off the 
shoulders on the shaft or file or trim off the ends 
of the bearings until the necessary end motion is 
obtained. 

(R31c) Then line up the shaft, pulley, and belt so 
that no end thrust is maintained on the shaft, but 
the armature has free end play while in motion. 

(32) Too great a load or strain on the belt. 

(R32a) Reduce the load so that the belt may be 
slackened and yet not slip. Avoid vertical belts iffl 
possible. (Vibration and flapping of belt causes 
lamps to flicker.) 



DISEASES or DIRECT-CURRENT DYNAMOS 287 

(R32b) Use larger pulleys, wider and longer belts. 
Run with slack side on top to increase the adhesion 
and pull of belt without excessive tightening. Belt 
should be tightened just enough to drive the full 
load smoothly without vibration or flapping. 

(33) Armature being not centrally located between the 
poles. 

(R33a) Bearing may be worn out, letting the arma- 
ture move out of center and should be replaced. 
See number (30). 

(R33b) Center the armature in the polar space and 
adjust the bearings to the new position. See 
number (30). 

(R33c) File out the polar space to give an equal 
clearance all around the armature. 

(R33d) Spring the pole away from the armature 
and secure it in place. This will be a difiicult if 
not an impossible job in large and rigid machines. 
322. Noise.— 

(34) The armature or pulley out of balance. 

(R34a) The armature and pulley should have been 
properly balanced when made. Their construction 
may, however, be somewhat improved by mounting 
them on knife edges and adding weight to the light 
side until balanced. 

(35) The armature strikes or rubs against the pole 
pieces. 

(R35a) Bend or press down and secure any project- 
* ing wires. Secure rigidly with proper tie bands of 

strong wire. 
(R35b) File out the pole pieces where armature 

strikes. See numbers (30) and (33) for possible 

remedies. 

(36) The collars or shoulders on the shaft, hub, or web 
of pulley strike or rattle against the bearing. 

(R36a) The bearing may be worn out and too loose 
and therefore rattle, new bearing needed. See 
numbers (30) and (33). 

(37) Loose screws, bolts, or connections. 

(R37a) Tighten up all the screws, bolts, and connec- 
tions to firm bearing and keep them so by daily 



PRACTICAL APPLIED ELECTRICITY 



attention. The jar and movement of dynamos 
tends to work screwed connections loose when not 
held by check nuts. 
(38) Singing or hissing of the brushes. See numbers 
(3), (4), and (5). 

(R38a) Apply a little mineral oil or better yet vase- 
line or hold a piece of stearic acid (adamantine) 
candle to the commutator and then wipe off. Just 
a faint trace of oil or grease is all that is needed. 

(R38b) Lengthen or shorten the brushes in the 
holder until firm yet gentle pressure is maintained 
free from any hum or vibration. See numbers (3), 
(6), (7), (8), (9), and (31). 
Flapping or pounding of the belts, joints, or lacing. 

(R39a) Use an endless belt. If a laced belt must be 
used, have square joints properly laced. 
Slipping of the belt due to an overload. 



(39) 



(40) 



See 



(R40a) Tighten the belt or reduce the load, 
number (32). 

(41) Humming of the armature lugs or teeth as they 
pass the pole pieces. 

(R41a) Slope the ends of the pole pieces so that 
the armature teeth do not pass the edges all at 
once. 

(R41b) Decrease the^ magnetism of the fields or 
increase the magnetic capacity of the teeth. 
323. Speed Too High. — 

(42) The engine fails to regulate under a varying load. 
(R42a) Adjust the governor to the proper regula- 
tion if possible; if not, get a better engine. The 
engine should regulate closely from "no load" to 
''full load" with the proper steam supply. 

(43) Series motor takes too much current for a given 
load and the motor runs away. (Load on the series 
motor is too small.) 

(A) Series motor on a constant-current circuit. 
(R43a) Put in a shunt and regulate until the proper 

current is obtained. 
(R43b) Use the proper regulator for controlling 
the magnetism of the field for a varying load. 

(B) Series motor on a constant-potential circuit. 



DISEASES OF DIEECT-CUERENT DYNAMOS 289 

(R44c) Put in a resistance to cut down the current. 
(R44d) Use the proper regulator or controlling 

switch. 
(R44e) Change to an automatic speed-regulating 

motor. 

(44) Shunt motor. 

(A) Regulator or field rheostat not properly set. 

(B) Voltage too high. 

(R44a) Adjust the regulator or field rheostat to 

control the speed. 
(R44b) Use the proper voltage and the proper 

rheostat. 

324. Speed Too Low. — 

(45) Same as number (42). 
(R45a) Same as (R42). 

(46) Overloaded. 

(R46a) See (Rlla) to (Rllg). 

(47) Short-circuit in the armature. 
(R47a) See (R12a) to (R12d). 

(48) Striking or rubbing of the armature on pole pieces. 
(R48a) See (R35a) and (R35b). 

(49) Friction. 

(R49a) See section (321). 

(50) Weak magnetic field. 

(R50a) See (RlOa), (RlOb) and (RlOc). 

325. Motor Stops. — 

(51) Too great an overload. See (D), (E), (F), and (G) 
under number (11). 

(R51a) Open the switch, locate the trouble, and re- 
move. Keep the switch open and the arm of the 
rheostat in the position ''off" while locating and re- 
pairing trouble. Then close the switch and move 
the arm gradually to the position "on" to see if 
everything is correct. With a series motor, no 
great harm will result from motor stopping or 
failing to start. If it is a shunt motor on a con- 
stant-potential circuit, the armature may and prob- 
ably will burn out or the fuse blow. 

(52) Very excessive friction. 

(R52a) Same as (R51a). See section (321). 



r 



290 PRACTICAL APPLIED ELECTRICITY 

(5.3) Circuit open. 

(A) Safety fuse melted. 

(B) Broke wire or connection. 

(C) Brushes not in contact. 

(D) Switch open. 

(E) Current fails or is shut off from the station. 
(R53a) Open the switch, locate and repair the trou- 
ble, and then put in the fuse. See (Rile). 

(R53b) Open the switch, locate and repair the trou- 
ble. See (13). 

(R53c) Open the switch to repair the fault. See 
number (3). 

(R53d) Close the switch, but before doing so see 
that the starting resistance is in circuit. 

(R53e) Open the switch, return the starting lever 
to the position "off," wait for the current, testing 
from time to time by closing the switch and moving 
the starting lever to the first closed-circuit position. 

(54) Complete short-circuit of the field. 

(R54a) Test for the fault and repair if possible. 
Inspect the insulation of the binding posts and 
brush holders. Poor insulation, oil, dirt, or copper 
dust may cause a short-circuit. See number (12). 

(55) Complete short-circuit of armature. 
(R55a) Same as (R54a). 

(56) Complete short-circuit of switch. 
(R56a) Same as (R54a). 

326. Motor Runs Backward or Against the Brushes. — 

(57) Wrong connections within the motor. 

(R57a) Connect up properly, referring to the proper 
diagram. If proper diagram is not to be had, try 
reversing the connections to the brush holders. 
Other changes may be made until proper connec- 
tions are found for rotation desired, then connect 
up permanently. 

327. Dynamo Fails to Generate. — 

(58) Reversed residual magnetism. 

(A) Reversed current in the field coils. 

(R58a) Send a current from another machine or 
from a battery through the field coils in the proper 



DISEASES OF DIRECT-CURRENT DYNAMOS 291 

direction to correct the fault. Polarity may be 
tested by a compass needle. If the connections of 
the windings are not known, try one and test; if 
not correct, reverse connections, try again, and test. 

(B) Reversed connections. 

(R58b) Connect up properly for rotation desired, 
referring to the proper diagram of the connections. 
See that connections for series coils (in compound 
dynamo) are properly made as well as those for the 
shunt coils. See number (57). 

(C) Earth's magnetism. 
(R58c) Same as (R58a). 

(D) Proximity of another dynamo. 
(RoSd) Same as (R58a). 

(E) Brushes not in the proper position. 

(R58e) Shift the brushes until evidence of an im- 
provement is given. The position of the brushes 
for best generating power should be clearly under- 
stood, and is generally at or near the neutral point. 

(59) Too weak residual magnetism. 
(R59a) Same as (R58a). 

(60) Short-circuit in the machine. 

(R60a) See numbers (12), (54), (55) and (56). 

(61) Short-circuit in the external circuit. 

(R61a) Lamp socket or other part of the line short- 
circuited or grounded may prevent building up of 
shunt or compound machines. Locate and remedy 
the fault before closing switch. See numbers (54), 
(55), and (56). 

(62) Field coils opposed to each other. 

(R62a) Reverse the connections of one of the field 
coils and test. Compass should show pole pieces of 
opposite polarity. If, after such a trial, dynamo does 
not build up, try (R58a). If the polarity does nol 
then come up in proper direction, cross the field 
connections or remagnetize them in the opposite 
direction. See number (58A). 

(63) Open circuit. 

(A) Broken wire. 

(B) Faulty connections. 

(C) Brushes not in contact. 



292 PRACTICAL APPLIED ELECTRICITY 

(D) Safety fuse melted or broken. 

(E) Switch open. 

(F) External circuit open. 

(R63a) Locate and repair brake. See number (13). 
(R63b) See (R58b). 
(R63c) See (R5a) and (R5b). 
(R63d) See (R53a). 
(R63e) Close the field switch. 

(R63f) Test out and repair with the dynamo switch 
open until the repairs are completed. 

(64) Too great a load on the dynamo. 

(R64a) Reduce the load to pilot lamps alone (shunt 
or compound machine). After dynamo comes up to 
full voltage, as shown by pilot lamps, or voltmeter, 
close the other circuits in succession and regulate 
the voltage at the same time. See (Rlla). 

(65) Too much resistance in the field regulator or field 
rheostat. 

(R65a) Gradually turn the regulating switch to cut 
out the resistance and watch the pilot lamp or volt- 
meter when the dynamo comes up to voltage, regu- 
late, etc. 
328. General Suggestions and Precautions. — Never use ice 
or water to cool off the bearings, as it may get into the 
armature and ruin it, unless it has been made waterproof as 
in the case of street-car motors. 

Never shut • down because of hot box until remedies for 
troubles (25), (26), (28), and (32) have been tried and proved 
useless. If absolutely necessary to shut down, get the belt 
off as soon as possible. Do not allow shaft to ''stick" in 
stopping. Get the boxes or bearings out and cool them off 
naturally as soon as possible and not in water, as this may 
ruin them. Then scrape, fit, polish, clean the shaft, and test 
for free turning by hand before belting up and starting again. 
Cleanliness about a dynamo or motor is imperative. Dirt, 
oil, or copper dust may prove a source of great annoyance or 
damage. Small tools, bolts, or pieces of iron must be kept 
away from a dynamo as they may be drawn into or fall upon 
armature and ruin it. It is always best not to allow loose 
articles of any kind to be placed upon any portion of a dynamo. 



li 



r 



DISEASES OF DIRECT-CURRENT DYNAMOS 293 

Brass or copper oil cans are best to use as they are non- 
magnetic. 

All the connections must be large, clean, and firm. Look 
over and tighten the loose connections, screws or bolts, daily. 

Always keep copper brushes raised from the commutator 
when the dynamo is at rest. Poor, cheap oil is poor economy. 
Use none but the best of mineral oils. All new oils should 
always be filtered before using. 

Keep cotton waste off the commutator. Use canvas or 
cloth, to wipe the commutator. 

A piece of pine makes a good burnisher to use on the 
commutator to keep it clean and smooth. 



CHAPTEE XV 

ELECTRIC LIGHTING 

329. The Electric Lamp. — The electric lamp in its broadest 
sense consists of a part of an electric circuit heated to incan- 
descence, together with special regulating and controlling 
devices. Electric lamps may be conveniently classified under 
three heads, viz, 

(a) Arc lamps (sections 330 to 333, inclusive). 

(b) Glow lamps (sections 334 to 339, inclusive). 

(c) Vapor lamps (sections 340 to 344, inclusive). 

330. The Carbon Arc Lamp. — If the ends of two carbon 
rods that are connected to some source of electrical energy 
be touched together and then drawn apart, there will be an 
electric arc formed between them. This arc consists of a 
column of very hot vapor which serves to conduct the elec- 
tricity from the end of one rod to the other. The terminals 
of the two rods will be wasted away as the arc continues to 
burn, the two rods, however, are not consumed at the same 
rate always. If the arc be formed by a direct current, the 
ends of the carbons assume a form similar to that shown in 
Fig. 238, the end of the positive becoming concave, and the 
end of the negative pointed. The carbon rod forming the 
positive terminal of the arc is consumed approximately twice 
as fast as the rod forming the negative terminal. The cup 
or concave portion of the positive carbon is called the crater; 
it is extremely hot and the greater part of the light the arc 
emits comes from this crater. The point of the negative 
carbon is not nearly so hot as the crater of the positive 
carbon, and it does not emit nearly so much light. As a 
result of this difference in light emitted by the positive and 
the negative terminals of the direct-current arc, the positive 
carbon will be placed above the negative so that the light will 
be cast downward, except in special cases; in the case 

294 



1 



ELECTRIC LIGHTING 



295 



of searchlights and projecting lanterns, the arc is arranged 
as shown in Fig. 239. If the electric arc be formed by an 
alternating current, the ends of both carbons are heated about 
the same, the upper carbon being perhaps a little hotter than 
the lower, due to the upward movement of the heated vapor. 
As a result of the heating of the ends of the two carbons 
being approximately equal, the light emitted from them will 
be approximately the same. The two carbons of an alternat- 





Fig. 238 



Fig. 239 



Fig. 240 



ing current are wasted away approximately the same, the 
upper one being consumed a little faster than the lower one. 
An alternating-current arc is shown in Fig. 240. The light 
emitted by the alternating-current arc lamp is not steady, but 
pulsates in value with the change in the value of the alter- 
nating current,- there being two pulsations of the light for 
each cycle of the current. 

A direct-current arc makes practically no noise when it 
is operating properly, while the alternating-current arc makes 
a loud humming noise on account of the continuous con- 
traction and expansion of the heated vapor, due to the varia- 
tion in the value of the current, and the lamp mechanism 
often vibrates, due to reversals in magnetism. 

A carbon arc lamp consists of two carbon rods, which have 
their distance apart controlled by a suitable mechanism that 
is operated by solenoids, or by hand. 

331. Regulation of Arc Lamps on Constant-Voltage and 
Constant-Current Circuit. — If an electric arc be connected 
directly to a constant-voltage line, it will be impossible to 
maintain the arc unless a resistance be connected in series 
with the arc for the following reason. Assuming the arc is 
operating satisfactorily on a constant-voltage line and that 
the current decreases in value, due to an increase in length 



296 PRACTICAL APPLIED ELECTRICITY 

Df the arc, caused by the ends of the carbon being consumed, 
this decrease in current will cause a decrease in the size of 
the arc unless an additional voltage be applied at the termi- 
nals of the arc, but the required increase in voltage is not 
available since the arc is operating on a constant-voltage 
circuit and, as a lesult, the lamp would go out almost in- 
stantly, or before the regulating mechanism has a chance 
to act and bring the carbons nearer together. When the 
circuit is once broken at the arc, the ends of the carbons 
must be placed in contact with each other and then drawn 
apart to re-establish the arc. 

If the current producing the arc is increased In value, 
there will be an increase in the size of the arc or a de- 
crease in its resistance and the constant voltage applied to 
the terminals of the arc would be more than sufficient to 
produce the desired current and, as a result, the current would 
continue to increase in value and would become excessive 
almost instantly, or before the regulating mechanism has 
time to operate and thus separate the carbons. 

By placing a resistance in series with the arc, an increase 
in current will cause an increase in drop over this resistance 
and a decrease in current will cause a decrease in drop. If 
the resistance is large enough in value, the change in the 
dfop over the resistance will more than balance the varia- 
tion in the drop over the arc caused by a change in the cur- 
rent, and the operation of the arc will be quite stable, A 
resistance used for the purpose just described is called a 
ballast. The ballast for alternating-current arc lamps is 
usually a reactance coil, instead of a resistance, the react- 
ance coil being formed by winding a coil on a laminated iron 
core. 

When arc lamps are operated on constant-current circuits, 
no ballast is required, as the current remains constant re- 
gardless of the position of the carbons, it being governed by 
the generator, transformer, or regulator. The drop in poten- 
tial over the arc, however, must be regulated in a series lamp 
by means of a solenoid which controls the distance between 
the carbons. 

332. Multiple Arc Lamps. — Multiple arc lamps require only 
one solenoid for their operation. When the lamp is iirst 
started, there is a large current in the circuit, the solenoid 



% 



ELECTEIC LIGHTING 



297 



is energized and draws the carbons apart until the decrease 
in current due to the increase in resistance of the arc weak- 
ens the solenoid to such an extent that it no longer acts to 
separate the carbons. If the current decreases in value, the 
solenoid is weakened, which allows the ends of the carbons 
to approach each other and the current is restored to its 
original value, or if the current increases in value, the solenoid 
acts to separate further the ends of the carbons and thus pre- 
vent an excessive increase in current. 
The circuit of a multiple direct-current 
arc lamp is shown in Fig. 241. The 
circuit is composed of the ballast or 
steadying resistance (R),the two regu- 
lating solenoids (Si) and (S2), and the 
arc (A) between the ends of the car- 
bons. The steadying resistance and 
the solenoids are so constructed that 
the number of turns in circuit can be 
varied by changing a connection from 
one tap to another on the various 
coils. The armature (B), controlled 
by the solenoids, actuates the upper 
carbon, the lower being stationary. 
The upper carbon is connected to the 
positive terminal of the circuit. 

333. Series Arc Lamps. — In this 
type of arc lamp no ballast or react- 
ance is required in series with the arc 
since the lamps are connected in 
series and operate on a constant-cur- 
rent circuit. The series lamp does not 

regulate automatically, as the multiple lamp does, and a 
second solenoid whose winding is connected in parallel with 
the arc is required. The mechanical action of this second 
solenoid is just opposite that of the solenoid connected in 
series with the arc. The diagram of a series arc lamp is 
shown in Fig. 242. When the lamp is first connected in the 
circuit, the two terminals are connected by means of three 
paths — one through the starting resistance (SR), and the cut- 
out (CO); a second through the series coil (C), adjusting 
resistance (AR) and arc in series; and the third through the 




Fig. 241 



298 



PRACTICAL APPLIED ELECTRICITY 



winding of the coil (DC), which is called the differential shunt 
on account of its winding being in shunt with the arc and its 
action differential with respect to the series coil. The two 
terminals of the lamp may be connected directly together 



by closing the switch 
top of the lamp. 



(S) which is usually located in the 



The operation of this lamp, 
which is typical of all series arc 
lamps, is as follows: The series 
solenoid is energized as soon as 
the lamp is connected in circuit, 
and the action of this solenoid 
will draw the ends of the car- 
bons apart, starting the arc and 
opening the cut-out (CO). As 
the carbons are drawn apart, the 
arc voltage increases and the 
current in the shunt solenoid 
(DC) increases, and its action 
tends to bring the carbons to- 
gether, but it does not overcome 
the action of the. series coil 
(SC), which tends to separate 
the carbons. As a result of 
these two solenoids acting at the 
same time, the arc is adjusted so 
that it takes a definite current, 
and a definite voltage exists 
across it. When the carbons are consumed, the shunt solenoid 
brings them nearer together, as its current has increased, due 
to the increase in voltage over the arc, and the current in the 
series coil remains approximately constant. This action con- 
tinues until the cut-out (CO) is closed and the current is 
shunted through it. The shunt solenoid (DC) is short-cir- 
cuited by the cut-out closing and there is no current in tl;e 
series solenoid (SC), the upper carbon is released ^nd the tvo 
come together. When the end ot the two carbons come to- 
gether, the current is re-established in the series solenoid and 
the lamp again picks up. If the carbons be entirely consumed, 
the cut-out remains closed and the circuit of which th«^ 




Fig. 242 



ELECTEIC LIGHTING 399 

lamp is a part is not opened, even though the lamp is not 
operating. A dash pot (DP) dampens the movement of the 
armature of the solenoids, which greatly improves the opera- 
tion of the lamp. In some types of series arc lamps the 
shunt and series windings are both placed on the same '. 
spool instead of on two different ones, as shown in Fig. 242. 

334. Glow Lamp. — In all glow lamps the light is emitted 
from a solid electrical conductor that is heated by an elec- 
trical current to such a temperature that it emits light. The 
lamps included under this head are the ordinary carbon-fila- 

. ment lamps, the different metal-filament lamps, such as 
tungsten and tantalum, and the Nernst lamp. 

335. Carbon-Filament Lamp. — The common carbon-filament 
incandescent lamp consists of a fine filament of carbon 
mounted in a highly exhausted glass bulb, which may assume 
a number of different forms, depending upon the particular 
use of the lamp. The carbon filament is connected to the 
external circuit by means of two short pieces of platinum 
that are embedded in the glass. Platinum is used for mak- 
ing these connections on account of it being capable of 
withstanding a high temperature and it contracts and ex- 
pands due to changes in temperature the same as glass, 
which is quite an advantage in maintaining the vacuum in 
the bulb containing the filament. 

The method usually employed in the manufacture of the 
carbon filament is as follows: Some of the fibrous sub- 
stances such as cotton are thoroughly cleansed and then dis- 
solved in a solution of zinc chloride by constant stirring, 
forming a thick fluid which is freed from lumps by filtering 
and from bubbles by heating almost to the boiling point under 
pressure. This fluid is then ''squirted" through a small hole 
and allowed to pass into an alcohol bath, which immediately 
hardens the gelatinous rod which can then be thoroughly 
washed in water. This thread has the appearance of celluloid, 
it is fairly,.tough, and may be cut into lengths and bent on 
formers intg any shape desired. They are then packed in 
charcoal and carbonized in a furnace, after which they are 
mounted on the platinum wires that are to form the terminals 
and heated electrically by a current to a high temperature in a 
vapor of hydrocarbon. The hydrocarbon vapor is decomposed 
by the hot filament, which results in a deposit of carbon on 



300 PRACTICAL APPLIED ELECTRICITY 

the filament. This deposit will be greatest at the hotest 
points in the filament, which will be at the points of highest 
resistance or smallest cross-section and, as a result, the fila- 
ment is made more uniform in cross-section, and its resist- 
ance slightly lowered, due to the increase in area. The above 
process is called flashing. The completed filament is placed 
in a glass bulb, which is exhausted by means of special 
pumps and sealed. A mounting suitable to screw into a 
lamp socket is then mounted on the bulb and the terminals 
of the filament connected to the two parts of the mount- 
ing, which make electrical contact with the two terminals of 
the line when the lamp is screwed into the socket. 

The resistance of a carbon filament when working under 
normal conditions is approximately half what it is at ordi- 
nary temperatures. 

336. Metalized Carbon-Filament or 
Gem Lamp.— The filaments used in the 
so-called metalized carbon-filament 
lamps are carbon filaments that have 
been subjected to an enormously high 
temperature, both before and after 
flashing, by means of a specially con- 
structed electric furnace. The term 
''metalized'' is used on account of the 
filaments acquiring a positive tempera- 
ture coefficient when subjected to very 
high temperatures. 

337. Tantalum Lamp.— The tantalum 
lamp was invented by Dr. Bolton — 
chemist for Siemens and Halske— who 
discovered the peculiarities of tantalum 
and the methods of obtaining the pure 
metal. Tantalum is obtained by special 

processes from the ores tantalite and columbite, the tantalite 
being found principally in Australia and the columbite in 
New England. Filaments of the desired size are drawn from 
the pure ductile tantalum. The melting temperature of tan- 
talum is approximately 2800° centigrade and its resistance 
increases with a rise in temperature. The tensile strength 
of tantalum is very high when cold, but when heated it be- 
comes soft and after the filament has been burning a few 




Fig. 243 



ELECTRIC LIGHTING 301 

hours it becomes very brittle, which is the great disadvan- 
tage of this kind of a filament. The filament of a tantalum 
lamp to operate on a 110-volt circuit is a great deal longer 
than the carbon filament, and special means must be provided 
for mounting those long filaments, such as shown in Fig. 243. 

The tantalum lamp gives a much whiter light than the 
carbon-filament lamp on account of the high temperature 
at which it operates. The life of a tantalum lamp is a great 
deal less on an* alternating-current circuit than it is on a 
direct-current circuit and its life on an alternating-current 
circuit depends upon the frequency. 

338. Tungsten Lamp. — The latest form of metallic-filament 
lamp placed on the market is the tungsten lamp. It has only- 
been commercially used for about three years, but even in 
that short time it has attained the highest position in the 
field of incandescent lighting. 

Tungsten is not a rare metal, having been used for a num- 
ber of years in the manufacture of tool steel and armor 
plate. Owing to the fact that as hitherto produced the metal 
was very hard and brittle, it has not until recently been 
obtained in a ductile form and consequently was not drawn 
into wire as was tantalum. It has a very high melting point 
— about 3050 °C. — which allows the lamps to be operated at 
the relatively high efficiency of 1.25 w.p.c. and even higher. 
This makes the light emitted very much nearer a pure white 
than any other incandescent lamp which has yet come into 
commercial use. 

There are four methods of forming the tungsten filament, 
but the one in most general use in this country is the Auer, 
or paste, process. This process starts with a pure tungstic 
acid, obtained from some one of the various tungsten ores, 
such as wolframite or scheelite. Tungstic acid is a yellow 
powder, which must undergo several processes of purifica- 
tion and reduction before it is finally reduced to the pure 
metal. This metal is used in the form of a fine black pow- 
der, which is mixed with a binding material in order to 
form a putty-like mass that can be squirted through a very 
small die, even as small as one-thousandths of an inch. A 
die is very expensive, being made from a diamond, through 
which a hole is drilled with a fine flexible steel needle. Its 
life is very short on account of the wear which the hard 



302 



PRACTICAL APPLIED ELECTRICITY 



metal gives the diamond and the enormous pressure of 30000 
pounds per square inch, under which the filament is formed. 
Thus only enough filaments for 1500 lamps can be formed 
before the die is so badly worn that it must be rebored for 
the next larger size filament, an operation which costs al- 
most as much as a new die. 

The filament while being squirted is looped back and forth 
on a card, which, when filled, is laid aside to allow the fila- 
ment time to dry. After drying, the filaments are cut into 
loops, which resemble in shape the finished filaments. These 
loops are placed in a tube furnace and baked at a cherry red 
heat in an atmosphere of gas which contains no oxygen. 
After baking for some time the more volatile constituents 
of the binder are found to have been driven off and the fila- 
ments are ready to be "formed." 
, They are now connected into an 

[^ L, ^^ electrical circuit by means of clips 

'^ ' *~^ and are gradually heated in an at- 

mosphere of inert gas, until incan- 
descence is reached, when the re- 
maining binding material is volatil- 
ized, leaving the pure metal fila- 
ment. This last process causes the 
filament to shrink somewhat, which, 
however, has been allowed for in the 
previous operations. 

The formed filament is now 
mounted on the center support and the two ends are welded 
to the supporting wires in an atmosphere of forming gas. 
After sealing the mount into the tubulated bulb, the lamp 
follows practically the same process of exhaustion as the 
tantalum lamp. 

There has been recently discovered a means whereby tungs- 
ten is rendered ductile, and wire-drawn filaments are the re- 
sult. Tungsten wire has a tensile strength varying from 
400 000 to 600 000 pounds per square inch, a value three to 
four tim.es that attained by tantalum wire. This wire can be 
bent into any form and consequently can be mounted in such 
a manner as to be more capable of withstanding shocks than 
was possible with a filament made by the paste process. 




GloweT 
Fig. 244 



ELECTEIC LIGHTINCr 



303 



HOLDING SCREW 1^ 

"~ ALUMINUM PLUQIa 

II ARMATMRE SUPPORT f O 

L.P08T>" 

-BALLWTl _ 

COT OUT COIL \< 

ARMATURE I 

^^j::::8i».ver contact stop I 

MOUSING I 

"contact SLEEVE PORCELAIN/ 
----GLOBE MOLDING SCREW 

•*• MOLOER PORCELAIN ),.j 

HEATER P0RCE»IN(3 

HEATER TUBEf O 

JI1_— . ^QLOWER ' I 



339. Nernst Lamp. — The filament or glower of a Nernst 
lamp consists of a small rod of porcelain-like material com- 
posed of the oxides of zirconium and yttrium. This rod is a 
good insulator at ordinary temperatures, but becomes a very 
good conductor at a low red heat, and its resistance decreases 
very rapidly as the temperature rises. If such a glower is 
to be used some means must be provided for heating it until 
it will become a conductor in starting the lamp, and a bal- 
last resistance must be con- 
nected in series with the 
glower to prevent the current 
becoming excessive after the 
glower has started to con- 
duct. A diagram of a single 
glower lamp is shown in Fig. 
244. When the lamp is first 
connected to the line, the cir- 
cuit is completed through the 
heater coil and cut-out. As 
the temperature of the glow- 
er is raised, due to the action 
of the heater, its resistance 
is lowered, which permits the 
electricity to pass through 
the glower, ballast, and cut- 
out coil all in series. After a short time this current will 
reach a value, due to decrease in the resistance of the glower, 
sufficient to operate the cut-out and thus disconnect the heater, 
and the glower will become incandescent. The ballast is com- 
posed of fine iron wire placed inside of an exhausted glass 
tube to prevent the iron oxidizing when it is heated. This 
ballast increases in resistance with an increase in tempera- 
ture, due to an increase in current and, as a result, automat- 
ically tends to maintain the current constant. A Nernst lamp 
is shown complete in Fig. 245. 

340. Vapor Lamp. — In the different forms of vapor lamp 
the light is emitted from an incandescent or luminescent 
column of vapor that conducts the electricity. The lamps 
included under this head are the flaming-arc lamps, mercury- 
vapor lamps, and the Moore tube. 




Fig. 245 



304 PRACTICAL APPLIED ELECTRICITY 

341. Flaming Arc. — In the carbon arc lamp the greater 
part of the light is emitted from the ends of the intensely- 
heated carbons, the arc itself emitting only a pale violet 
light. If the carbons between which the arc is formed be 
impregnated with metallic salts, or rods of metal, or metal 
oxide be used instead of carbon, an arc will be formed which 
is highly charged with metal vapor. The light emitted by- 
such an arc comes mostly from the incandescent particles in 
the metal vapor, very little coming from the terminals ot 
the rods forming the arc. Arc lamps employing impregnated 
carbons or metal rods instead of carbons to form the arc 
are known as flaming arcs. 

One of the materials most commonly used in combination 
with carbon is calcium, which gives the light emitted by the 
arc a highly brilliant yellow color. Carbons impregnated 
with stroncium give a red light, and those impregnated with 
titanium or barium give a white light. There are many 
different forms of electrodes for the flaming arc lamps, con- 
sisting of different metals and combinations. 

342. Bremer Flaming-Arc Lamp. — The Bremer lamp is one 
of the oldest and is the most eflicient of the different flam- 
ing arc lamps on the market. The carbons in this lamp are 
placed parallel or slightly inclined to each other, instead of 
being placed in the same plane with their axes coinciding, 
which results in a larger arc being formed between them. 
This arrangement of the carbons gives the lamp an additional 
advantage over the other arrangement, since there is no 
lower carbon in the path of maximum illumination. In the 
construction of the lamp, a magnet is provided which creates 
a magnetic field that acts on the arc and deflects it down- 
ward, thus preventing its climbing up the carbons, and at the 
same time giving a longer and larger arc and a better 
distribution. 

343. Mercury-Vapor Lamp. — The mercury-vapor lamp con- 
sists of a highly exhausted glass tube with two platinum 
wires sealed in its ends. One of these wires is connected 
inside the tube to a piece of iron or graphite that forms the 
anode and the other is connected to a pool of mercury that 
forms the cathode. In starting such a lamp a high electro- 
motive force or a special starting device is required, but 
after the lamp is once started a current of several amperes 



ELECTRIC LIGHTING 



305 



can be easily maintained by a pressure of from 30 to 100 
volts. The mercury vapor in the tube gives off a bright 
light that is deficient in the longer wave-lengths, or red. The 
lack of the longer wave-lengths results in an unpleasant 
distortion of color. 

The lamp may be started by tilting the tube, it being nor- 
mally at an angle to the horizontal, which allows the mercury 
to extend from one end to the other and complete the cir- 
cuit, and the circuit is maintained through the mercury vapor 
when the metallic mercury circuit is broken by the tube be- 




Fig. 246 



ing allowed to return to its normal position. A spark may be 
caused to pass through the tube, due to the action of an in- 
duction or kick coil which ruptures the medium and starts 
the lamp. A third means of starting is to place an auxiliary 
positive electrode near the negative electrode to be used in 
starting and the connection is then transferred to the other 
positive electrode after the lamp is started. 

The lamp is operated on alternating current by connect- 
ing the negative electrode of the lamp to the middle point 
of a transformer winding and having two positive electrodes 
located at the same end of the tube and connected to the 



306 



PRACTICAL APPLIED ELECTRICITY 




two ends of the transformer winding. A mercury-vapor lamp 
is shown complete in Fig. 246. 

344. Moore Tube. — The Moore tube resembles the mer- 
cury-vapor lamp in that it consists of a conducting vapor en- 
closed in a glass tube and the light is emitted from incan- 
descent particles in the vapor stream. The tube from which 
the light is emitted may be of any length up to about 200 
feet, of any desired form, and its diameter may be as large 
as 1% inches. Two graphite electrodes are sealed in the 
ends of this tube, which is filled with air, carbon dioxide, 
nitrogen, or other suitable gas. The secondary winding of a 





Fig. 247 



Fig. 248 



transformer is connected to the graphite terminals in the 
ends of the tube, the primary winding being connected to 
the source of power, as shown in Fig. 247. The vacuum in 
the tube becomes more and more perfect, which decreases 
the efficiency of the lamp, and some means must be provided 
for regulating the vacuum, which is done as follows: A 
separate tube (t) projects downward from the main one, 
as shown in Fig. 247, and is connected with what is called a 
feeder valve. This feeder valve, shown more in detail in 
Fig. 248, consists of a solenoid connected in series 
with the primary winding of the transformer. The iron 
plunger (I) is fastened in a glass displacement tube and it 
is raised or lowered, due to a change in the current in the ■ 
solenoid. In the end of the tube (t) there is cemented a 
carbon plug with a very small opening in it, which is nor- 



ELECTRIC WIRING 307 

mally covered with mercury. If the displacing tube be moved 
up, due to the action of the solenoid, the mercury recedes, 
uncovering the opening in the carbon plug and allowing a 
small amount of gas to enter the tube. 

With an increase in vacuum there is a decrease in resist- 
ance and an increase in current in the secondary winding 
of the transformer, and also a corresponding increase in 
current in the primary winding. This increase in current 
causes the feeder valve to operate and the vacuum is re- 
stored to normal by admitting gas. The secondary voltage 
required will depend upon the length of the main tube. 

345. Units of IMumination. — The luminous intensity of a 
source of light is measured by comparing it with a source of 
unit intensity, and the unity commonly employed for meas- 
uring luminous intensity is the candle-power. The unit of 
candle-power is equal to 1.111 hefner units. 

The Hefner lamp, so-called from its inventor, is a specially 
constructed lamp that burns pure amyl acetate. The wick 
and the tube holding the wick are of definite dimensions and 
in using the lamp the wick is adjusted so that the flame is 
a prescribed height. The intensity of a beam of light in a 
horizontal plane from such a lamp is called a hefner unit or a 
hefner. 

The illumination or the amount of light falling on an ob- 
ject is measured in a unit called the foot-candle. A foot- 
candle is the normal illumination produced by one unit of 
candle-power at a distance of one foot. 

The mean horizontal intensity is the average intensity in 
all directions in a horizontal plane that passes through the 
source of light. In the case of incandescent lamps this plane 
is taken perpendicular to the axis of the lamp. 

The mean spherical candle-power is the average candle- 
power taken in all directions around the source of light. 

346. Photometry. — The measurement of light emitted by 
a lamp is called photometry. This is always accomplished 
by comparing the beam of light from a given lamp with the 
beam of light from a standard lamp, and the device used for 
making this comparison is called a photometer. The inten- 
sity in different directions can be measured by placing the 
lamp in different positions with respect to the photometer. 

Lighting measurements are all based upon the law of in- 



308 



PRACTICAL APPLIED ELECTRICITY 



verse squares, which can be explained by reference to Fig. 
249. A source of light is shown at (L) and four rays of 
light (Ri), (R2), (R3), and (R4) are shown emanating from 
(L) and forming a pyramid. Three cross-sections through 
this pyramid and perpendicular to its axis are shown at (A), 
(B), and (C). These three cross-sections and the light (L) 
are all equally spaced and, as a result, the cross-section (B) 
is four times that of (A), and (C) is nine times that of (A), 
etc., or the cross-sections between the four rays increase as 
the square of the distance the cross-sections are from the 
source of light. Since the sam.e total number of rays pass 
through each area, the intensity per unit of area varies in- 



^ 



I 







4^ 



Fig. 249 



Fig. 250 



versely as the square of the distance from the source of light. 

The Bunsen photometer is one of the oldest and simplest 
forms of photometers and also one of the most eflScient means 
of comparing the intensities of different sources of light. The 
construction of this photometer can best be explained by ref- 
erence to Fig. 250. A sheet of white paper, the center por- 
tion of which is made transparent by being treated with 
paraffin or some other similar substance, is mounted inside 
a box (B), as shown in the figure. This sheet of paper is 
called the screen (C) of the photometer. 

The two sides of this screen are viewed simultaneously 
through the sight piece (S) by the aid of the two mirrors 
(Ml) and (M2) which are so mounted that they make an angle 
of about 140 degrees with each other and equal angles with 
the plane of the screen. The light falling on either side of 



I 



ELECTRIC LIGHTING 309 

this screen is not all reflected, a part passing through the 
translucent spot in the center. When the illumination on 
both sides of the screen is the same, an equal amount of 
light is transmitted through the screen in both directions, 
and if the lights on the two sides are of the same hue the 
spot in the center and the remainder of the screen should 
appear of the same color. 

The photometer is used as follows: Two lamps, whose in- 
tensities are to be compared, are located a known distance 
apart on what is known as the photometer bench. The box 
(B), Fig. 250, is placed between the two lamps so that its 
axis (X1X2) coincides with the line connecting the cen- 
ters of the two sources of light and so arranged that it can 
be moved along this line. It is then moved until the two 
parts of the screen have the same appearance. When this 
balance is obtained, the distance the center of each source 
of light is from the screen is determined and the relation 
of the intensities is calculated by applying the law of inverse 
squares given above. If the candle-power of one lamp is 
known, the candle-power of the other one can be easily de- 
termined, as follows: 

Let (Ss) represent the candle-power of the standard lamp. 

Let (Sx) represent the candle-power of the lamp being 
tested. 

Let (Ds) represent the distance the screen is from the 
standard lamp. 

Let (Dx) represent the distance the screen is from the lamp 
being tested. 

Sx Ds2 
Then =^ 



Ss Dx2 

Ds2 

and Sx = Ss (126) 

Dx2 

347. The Specific Consumption of Lamps. — The specific 
consumption of a lamp in practice is always specified by giv- 
ing the watts consumed in the lamp per spherical candle- 
power of light emitted. The specific consumption of different 
types of lamps is given in Table X. 



310 PRACTICAL APPLIED ELECTRICITY 

TABLE NO. X. 



1 



APPROXIMATE SPECIFIC CONSUMPTION OF DIFFERENT TYPE 

LAMPS 

Watts per Mean 
Spherical Candle- 
Tj^pe of Lamp — Power Remarks 

Direct-current series 1.0 

Direct-current multiple 2.4 

Alternating-current series... 1.7 No outer globe 

Alternating-current multiple.2.5 No outer globe 

D. C, Bremer flaming arc... .196 48 volts over arc 

A. C. Bremer flaming arc... .226 48 volts over arc 

Carbon-filament lamps 3.0 to 3.5 

Tantalum lamps 1.8 to 2.2 

Tantalum lamps 1.8 Direct current 

Tungsten 1.25 

Nernst 2.95 - Six glower. 

Nernst 3.92 Single glower 

Mercury-vapor 48* See note (A) 

Moore tube 1.7* See note (B) 

Note (A) : The candle-power is measured perpendicular 
to the axis of the tube. 

Note (B) : This value corresponds to a vacuum in the 
tube of approximately .10 millimeters of mercury. 

348. Distribution Curves. — The distribution of light from 
a source through any plane and in different directions can 
be represented graphically, and such a graphical representa- 
tion is called a distribution curve. The distribution curve, 
about a four-candle-power lamp in a vertical plane, is shown 
in Fig. 251. The intensity in different directions being pro- 
portional to the distance, the curve is from the center. The 
form of the filament, bulb, shades, and reflectors all have an 
influence in determining the shape of the distribution curve. 

349. Calculation of Illumination. — If a 16-candle-power 
lamp (L) be placed 8 feet from the plane (A), as shown in 
Fig. 252, the illumination per unit area on the plane (A) at 
the point (B) would be equal to (16-7-82) = i^ toot-candle, 
since the intensity varies inversely as the square of the 
distance. In general, to get the illumination at any given 



*Watts per candle-power. 



ELECTRIC LIGHTING 



311 



point, the candle-power in the direction measured must be 
divided by the square of the distance from the light to the 
point illuminated. 

When the surface illuminated is not at right angles to the 
light, the values of illumination obtained above must be mul- 
tiplied by a reduction factor, which takes into account the 
angle at which the rays of light strike the surface, Thus, 




Fis. 251 

if a lamp (V) be placed 8 feet above the horizontal plane 
(H), as shown in Fig. 253, and it is desired to determine 
the illumination at the point (B), 6 feet from the vertical 
line through the lamp, you should proceed as follows: The 
distance (I) the point (B) is from the lamp is equal to 

I = •\/v2 + h2 

The value of (Z) in Fig. 253 is equal to 



^ = V82 + 62 = 10 

Assuming the intensity of the light from the lamp (L) along 
the line (l) is 20 candle-power, then the illumination on a 
surface (P) at the point (B) perpendicular to the line 
(I) would be equal to 

20 1 

Illumination = = — foot-candle 

102 5 



312 



PRACTICAL APPLIED ELECTRICITY 













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ELECTKIC LIGHTING 



313 



The illumination of i^ toot-candle on the surface (P) is to 
be distributed over a larger area when the horizontal plane 
is considered. The ratio of the illumination per unit of area 
on the horizontal plane to the il- 
lumination per unit area on the 
plane (P) is equal to the inverse 
ratio between a unit area in the 
plane (P) and the projection of this 
unit area on the horizontal plane. 
This ratio is equal to the cosine of 



-8 Feet 



B 



Fig. 252 




Fig. 253 



the angle between the plane (P) and the horizontal plane, 
which is the same as the angle between (I) and a vertical 
line through the lamp. The constants by which the intensity 
in candle-power along the line (0 must be multiplied to 
obtain the illumination on the horizontal for different values 
of (h) and (v) are given in Table No. XL The illumination 
on the horizontal plane for the above example would be 

equal to 

20 X .008 015 = .1603 foot-candle 
When the source of light consists of a number of lamps, the 
illumination for each lamp can be determined and the re- 
sultant illumination obtained by addition. 

TABLE NO. XII 
REQUIRED ILLUMINATION FOR VARIOUS CLASSES OF SERVICE 

Intensity of 
Illumination 
Class of Service in Foot-Candles 

General illumination of residences 1 to 2 

Reading 1 to 3 

Auditoriums and theaters 1 to 4 

Churches 2 to 4 

Bookkeeping and clerical work 3 to 5 

General illumination of stores 2 to 5 

Engraving and drafting 5 to 10 

Street lighting by» electricity 05 to .06 



314 



PRACTICAL APPLIED ELECTRICITY 



The illumination required for various classes of service is 
given in Table No. XII. 

350. Shades, Reflectors, and Diffusers. — Often the distribu- 
tion from a source of light is undesirable and the intrinsic 
brilliancy of the source may be too great for the best effect 



270' 
255 

240 



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210- 


195° 


l< 


30" 


165° 


150' 



90* 
105' 
120* 

135* 



Fig. 254 



or greatest comfort, which results in the use of shades, re- 
flectors and diffusers. A shade is used to modify the light 
and is placed between the source and the eye. It may act 

as a reflector or diffuser. A 
reflector serves to re-direct 
the light and thus change 
the distribution, and it may 
act as a shade in certain 
directions. A diffuser is 
intended to decrease the 
intrinsic brilliancy and to 
reduce the glare, and in so 
doing act as a shade or 
reflector. The arc lamp is 
usually enclosed in two 
globes when used for in- 
terior lighting, in order to reduce the intrinsic brilliancy. The 
change in shape of the distribution curve, shown in Fig. 251, 
due to the use of a holoplane reflector, is shown in Fig. 254. 
The reflector and lamp are shown in Fig. 255. 

351. The Effect Different Colored Walls Have Upon the 
General Illumination. — In rooms not larger than approxi- 




Fig. 255 



ELECTRIC LIGHTING 315 

mately 15 feet by 15 feet, the color of the walls will have 
considerable effect upon the general illumination when the 
lamps are used without shades or reflectors. The effective 
illumination in rooms with different colored walls is obtained 
by multiplying the illumination given directly by the lamp 
by the factor given in Table No. XIIL 

TABLE NO. XIII 

FACTORS FOR OBTAINING EFFECTIVE ILLUMINATION 

Illumination 
Color Wall Factor 

White paper 3.3 

Orange paper 2.0 

Yellow paper 1.67 

Yellow painted wall 1.67 

Pink paper (light) 1.56 

Green paper (emerald) 1.22 

Brown paper (dark) 1.15 

Blue-green paper 1.14 

Chocolate paper (deep) 1.04 



CHAPTER XVI 

ELECTRIC WIRING 

352. Wiring in General. — The term ''electric wiring" is 
usually considered to mean the proper placing of electrical 
conductors to form a complete metallic circuit for conducting 
the electrical energy to and from the points where it is gen- 
erated and consumed. The above use of the term, however, 
is rather narrow and it should include in addition to the 
proper placing of the electrical conductors, the calculation or 
the proper size of conductors to meet given requirements; the 
selection of the proper materials; a consideration of the 
proper location of the service mains, meters, outlets for 
lamps, panel boards, switches, controlling devices, etc., and a 
thorough understanding of the requirements of the inspection 
department under whose jurisdiction the work is being done. 

353. Factors Determining the Size of Conductors. — The 
problem in electric wiring is the transmission of a certain 
amount of energy from one point to another, and it is usually 
desired that this transmission be effected with a minimum 
loss and at the same time keep the initial cost within a rea- 
sonable value. 

The drop in electrical pressure between any two points 
connected by an electrical conductor is equal to the product 
of the current in the conductor connecting the two points and 
the resistance of the conductor, or volts drop equals (R X I). 
This drop in pressure in volts multiplied by the current in 
amperes gives the value of the power loss in watts in the 
conductor, or power loss in watts equals (R X I) times (I), 
which equals (I2R). The current in a conductor forming part 
of the circuit connecting a given load to its source of energy 
is fixed by the voltage of the generator and the resistance 
of the load. Since the value of the current in the expression 
for the power loss in a conductor is to be fixed for any given 

316 



ELECTKIC WIKING 317 

voltage, the only way to reduce the loss Is to reduce the value 
of the resistance (R). 

The resistance of a conductor composed of a given material 
and at a constant temperature varies directly as the length 
and inversely as the cross-sectional area. The length of a 
conductor connecting a load and generator is usually deter- 
mined by the distance between them (in some cases the 
conductor may be a great deal longer than this distance on 
account of its not following a direct path), and, as a result, the 
resistance can be reduced only by an increase in the cross- 
section of the conductor. An increase in cross-section, the 
length remaining constant, means a like increase in weight, 
and hence an increase in cost. The loss in the conductors 
in any circuit may be made very small by an increase in their 
cross-sections, but, as pointed out above, the cost of the con- 
ductors is increasing at the same time the loss is decreasing. 
In practice, a reasonable loss, usually expressed in per cent, 
is allowed and the size of the conductor required to carry the 
current is figured from this value. 

Neglecting the loss in the conductors, there is another 
important factor to consider in the determination of the 
proper size of conductors to use, and that is the allowable 
rise in temperature of the conductor caused by the loss in 
it. The rise in temperature due to a given loss will depend 
upon the rate at which the heat generated in the conductor 
is radiated, which in turn will depend upon its insulation and 
mechanical protection. The allowable current-carrying ca- 
pacity for different size copper wires has been established 
by the Underwriters, and Table G, Chapter 20, gives a 
complete set of these values. It must be understood that 
the percentage drop in voltage is not taken into account in 
this table. 

In some cases the size of the conductor is determined by 
the mechanical requirements; it, however, should never be 
smaller than the size determined by the electrical require- 
ments. 

354. Choice of Material to Use as a Conductor. — From the 
standpoint of loss in the line, the choice of a material to use 
as a conductor is governed by its resistance. In order that 
the loss be small, the specific resistance must be low. This, 
however, does not mean that the materials of lowest specific 



318 PRACTICAL APPLIED ELECTRICITY 

resistance will be used, as their cost may be a great deal 
more than that of some material having a higher specific 
resistance. If the specific resistance of a material which we 
shall call (A) is twice that of a material which we shall call 
(B), and the cost of the material (A) per unit volume is 
just one-half that of material (B), then the cost of the two 
conductors, one each of the (A) and (B) materials, of the 
same length and having the same resistance, will be the same. 
As far as the loss in the conductors and the initial cost are 
concerned, there would be no choice in the above case. The 
conductor composed of material (A) would be larger than 
the one composed of material (B), and more material would 
be required to insulate it; a larger conduit would be required 
to accommodate it if it be placed in a conduit; and it would 
be bulkier and more than likely harder to handle. The tensile 
strength of one material might be such that it would not 
meet the mechanical requirements although it fulfills the 
electrical requirements. Thus a long span across a stream 
w^ould be made of steel wire on account of its high tensile 
strength and not for electrical reasons. The material that 
seems to meet best all the requirements for a conductor, 
except in special cases, is copper, and for this reason copper 
is usually used. The supply of copper and its cost are also in 
its favor as compared to silver, which is electrically better. 

355. Calculation of the Resistance of a Conductor. — The 
resistance of any conductor (neglecting changes in tempera- 
ture) can be determined if its dimensions and the value of the 
specific resistance of the material composing it are known. 
The equation used in making this calculation is 

Kl 

R = (127; 

d2 

In the above equation (K) represents the mil-foot resistance, 
or it is the resistance of a portion of the conductor one foot 
in length and having a circular cross-section one-thousandth 
of one inch in diameter; (l) is the length of the conductor 
in feet; and (d) is the diameter of the conductor in mils (the 
mil is equal to the one-thousandth part of one inch). 

Example. — Calculate the resistance of a conductor, com- 
posed of commercial copper having a mil-foot resistance of 



ELECTEIC WIRING 319 

(10.8), 500 feet in length, and having a diameter of 325. mils. 
(This corresponds to a No. 0, B. & S. gauge wire). 

Solution. — Substituting in equation (127), the values of (K), 
(I) and (d) given in the problem, gives 
10.8 X 500 

R = = .05112 

325 X 325 

Ans, .05112 ohm. 
If the length of a circuit is given, the value of {I) to sub- 
stitute in equation (127) is equal to twice the length of the 
circuit; or if the length of the circuit be represented by the 
letter (L) the value of (0 is equal to (2L). 

356. Calculation of Size of Conductor When Allowable Drop 
and Current Are Given. — If the drop in potential (E), in volts, 
that is to occur in any circuit when there is a definite cur- 
rent in the circuit, and the current of (I) amperes are both 
given, the resistance of the circuit in ohms is equal to 

E 
R = — (128) 

I 
Knowing the resistance of the circuit and its length, the size 
of the conductors required can be determined by substituting 
in the following equation: 

I X K X 2L 

d2^ (129) 

E 
Since from equation (128) 

E 



and from equation (127) 



R: 



we have 



K X 2L 

d2 

E K X 2L 

I d? 

or 

Ed2 = I X K X2L 



320 PRACTICAL APPLIED ELECTRICITY 

and 

I X K X 2L 

d2 = ' 

E 
Example. — The voltage at the terminals of a generator is 
112 volts, and it is desired to transmit 50 amperes over a 
circuit 450 feet in length with a drop in voltage not to exceed 
two per cent. What size of conductor is required if it be 
composed of copper having a mil-foot resistance of 10.8? 

Solution. — The drop in voltage (E) is equal to two per cent 
of 112, or 

E = .02 X 112 = 2.24 volts 
Substituting the value of (E) just obtained, and the value of 
(I), (L), and (K) in equation (129), gives 
50 X 10.8 X 2 X 450 

d2 = 

2.24 
Solving this equation, gives 

d2 = 215 170 
and 

d = 464 mils 

Referring to a table giving the diameters of different size wire 
you will find a No. 0000 B. & S. gauge is the size having 
a diameter practically the same as the value just obtained. 
In case the value of (d) obtained is less than the diameter of 
a No. 14 wire, the No. 14 wire should always be used, or in 
no case use a wire smaller than No. 14, B. & S. gauge, except 
in the wiring of fixtures. 

Tables L, M, and N, Chapter 20, give the proper size 
conductors to use on 50-, 110-, and 220-volt circuits, when 
the distance to the center of distribution and the current the 
conductor is to carry are given, the loss in each case being 
two per cent. The following example will illustrate the use 
of the tables: 

Example. — A current of 100 amperes is to be supplied to a 
number of incandescent lamps from a generator whose ter- 
minal voltage is 220 volts with a loss not to exceed two per 
cent. What size conductor should be used when the gen- 
erator and lamps are 200 feet apart? 

Solution. — Referring to Table N, Chapter 20, and pass- 



ELECTEIC WIEING 321 

ing along the horizontal line corresponding to 100 amperes 
until you strike the vertical column headed 200, which cor- 
responds to the distance to the center of distribution, you 
will find that a No. B. & S. gauge wire is the proper size 
to use. 

In some cases the percentage loss to be allowed in the line 
may be different from that given in the tables, and the size 
wire can be determined as follows: The percentage loss 
allowed should be divided by two, giving a constant which 
we shall call (C). Then determine the size wire required by 
means of the tables, on the assumption that the loss is two 
per cent, divide the circular-mil area of the wire thus obtained 
by the constant (C), and look up the size of wire having a 
circular-mil area corresponding to or next larger than this 
result. 

Example. — It is desired to transmit 100 amperes from a 
220-volt generator a distance of 200 feet with an allowable 
loss of five per cent. What size wire should be used? 

Solution. — The constant (C) is equal to 5 divided by 2, or 

C = 5--2 = 2.5 

Referring to Table N, Chapter 20, we find that a No. 00 
B. & S. gauge wire would be required if the loss were only 
two per cent. The circular-mil area of a No. 00 B. & S. 
gauge wire is, from Table C, equal to 133 100, and this value 
divided by (C), or 2.5, gives the circular-mil area of the wire 
to be used. 

Circular-mil area -= 133 100 ~ 2.5 = 53 240 
diameter = 230.7 mils 

A No. 3 B. & S. gauge wire has a diameter of 229.4 mils, 
so that it should be used. 

357. Motor Wiring Formula. — The size of wire required to 
supply energy to a direct-current motor can be calculated 
as follows: Assuming the horse-power, efficiency, and the 
voltage of the motor are given, the current that must be sup- 
plied when the motor is operating under full load can be 
determined by substituting in the following equation: 

h.p. X 746 100 

1 = X (130) 

E F 



322 PRACTICAL APPLIED ELECTRICITY 

In the above equation (h.p.) represents the horse-power of 
the motor, (E) the voltage the motor is designed to operate 
on, (F) the efficiency of the motor in per cent, and (746) is 
the number of watts per horse-power. Having determined 
the value of the current (I), the size of wire required can be 
determined as in section (356). The size of conductor used 
should be such as to allow for a 25 per cent overload on the 
motor, the current in the conductor not exceeding the values 
given in Table G, Chapter 20. 

Example. — A 50-horse-power direct-current motor is to be 
supplied with current from a 220-volt generator over a line 
250 feet in length and having such a resistance that the drop 
in voltage shall not exceed 4 per cent. What size wire should 
be used, the efficiency of the motor being 90 per cent? 

Solution. — Substituting directly in equation (130) gives 

50 X 746 100 

I = — ' X = 188. + amperes 

220 90 

When the motor is carrying a 25 per cent overload the cur- 
rent in the leads will be equal to 1.25 times 188, or 

Overload current — 1.25 X 188 = 235 amperes 

Since the maximum value of current given in Table N, 
Chapter 20, is less than 235 amperes, the size wire can be cal- 
culated as follows: The size of wire required to carry 235 
amperes with a 4 per cent drop will be the same as the size 
of wire required to carry 117.5 amperes with a 2 per cent 
drop. Referring to Table N, we find, that with a current of 
120 amperes and a distance of 250 feet, a No. 000 B, & S. 
gauge wire is required. Referring to Table G, we find that 
No. 000 B. & S. gauge wire cannot be used to carry 235 am- 
peres except when insulations other than rubber are used. 
The size of wire required, if it is insulated with rubber, would 
be a 300 000 circular-mil cable. 

358. Methods of Wiring and Rules Governing Same. — The 
rules and requirements governing the proper installation of 
electrical conductors and the kind of material that must he 
used vary in different localities. In the majority of cases 
the rules and requirements of the National Board of Fire 
Underwriters as published in the National Electric Code are 



J 



ELECTKIC WIRING 323 

followed. In some municipalities there are certain ordinances 
in force, or the power company may have certain require- 
ments that must be met before they will supply electrical 
energy. The National Electric Code should, however, be fol- 
lowed at all times unless there are other requirements in 
force. 

There is a supplement to the National Electric Code that 
contains a complete list of all the fittings that are approved 
by the Underwriters, and in no case should fittings be used 
that do not appear in this supplement unless by special per- 
mission. 

There are several methods of wiring that are approved by 
the National Board of Fire Underwriters, viz.: 

(a) Open, or exposed, work. 

(b) Moulding work. 

(c) Concealed "knob and tube" work. 

(d) Interior conduit and armored cable work. 

359. Open, or Exposed, Work. — This method of wiring is 
perhaps the cheapest of any of the methods given, and it is 
at the same time one of the safest and best methods when 
properly installed. It is used a great deal in mills, factories, 
etc., where the appearance of the wires on the ceiling or walls 
is of no great importance. The wires used in this method 
of wiring may be either rubber covered or provided with 
slow-burning weatherproof insulation. Rubber insulation 
should always be used when the wire is in a damp place, such 
as a cellar, and either weatherproof or rubber insulation may 
be used to protect it against corrosive vapors. When installed 
in dry places and for voltages below 300, the wires should 
always be rigidly supported on non-absorptive, non-combustible 
insulators that separate them 2i/^ Inches from each other 
and ^ inch from the surface over which they pass. For 
voltages from 300 to 500, the wires should be separated 4 
inches from each other and 1 inch from the surface over 
which they pass. If the wiring is in a damp place, the wires 
should always be at least 1 inch from the surface over 
which they pass, even if the voltage be below 300 volts, the 
other distances being the same as above. The wires should 
always be protected by porcelain tubes or short pieces of 
flexible tubing when they pass over pipes or other material. 



324 



PRACTICAL APPLIED ELECTRICITY 



as shown in Fig. 256. It is usually best to place the wir 
above the pipes rather than under them. The wires, when 
they run vertically on the walls, should be protected by 
suitable boxing or run in a pipe, as shown in Fig. 257. Ther 
should be a clearance of at least 1 inch around the wirei 
when they are placed inside the box, which should be closed at 
the top, and the holes in the top of the box where the wires enter 
should be bushed with porcelain tubes. * If the wires be placed 
inside a pipe, they should each be encased in a piece of flexible 



i 

^ires 
n 

i 



^^QasPipe 



Fig. 256 




Fig. 257 



tubing that will extend from the insulator below the end of the 
pipe to the first one above it. Porcelain tubes should always 
be used when the wires pass through walls and floors. These 
tubes should be long enough to reach all the way through 
the wall or floor, except in special cases, when a piece of pipe 
may be used with a long porcelain bushing put into it from 
each end and cemented in place. 

360. Moulding Work, — In this class of work the wires are 
placed in grooved pieces of wood that are provided with a 
wooden cap that is fastened to the body of the moulding after 
the wires have been put in place. The dimensions of this 
moulding are governed by the requirements of the Under- 
writers, and the number of grooves is usually two or three, 
depending upon whether it is to be used on a two- or three- 
wire system. The capping can be made to correspond in 
design and color to the other woodwork in a building or 
room, which makes it less conspicuous. 



ELECTRIC WIRING 



325 



Moulding work is extensively used alone and in combination 
with other classes of work, there being numerous fittings on 
the market that can be used in changing from one system to 
the other. It cannot be used in damp places, or in rooms 
where it will be subjected to fumes, or in concealed work or 
elevator shafts. Approved rubber covered wire should be 
used when it is placed in moulding. The same precautions 
should be taken in protecting the wires in this class of work 
where they pass through floors and ceilings as were men- 
tioned for open, or exposed, work. A device known as a 
kicking box, Fig. 258, is usually used in protecting the wires 
at the points where they enter or emerge from the floor, 




Moulding 



loor 




Fig. 258 



Fig. 259 



Meta;i mouldings, such as those shown in Fig. 259, are used 
quite extensively at the present time, on circuits requiring 
not more than 600 watts and where there is a difference in 
potential not to exceed 300 volts. Special fittings must be 
used with this kind of moulding so that it is continuous both 
mechanically and electrically. The moulding should be 
grounded, or the rules governing its installation are prac- 
tically the same as those governing the installation of conduit 
work. 

361. Concealed "Knob and Tube" Work. — Concealed **knob 
and tube" work is fast disappearing on account of the great 
danger of fire due to short-circuits between the wires that 
are placed in the walls and floors.. This class of wiring is 
quite cheap and it is extensively used in frame buildings in 
localities where the Inspection Departments will permit. The 
wires for this class of work must have an approved rubber 
insulation and be supported by means of knobs or tubes on 



326 



PRACTICAL APPLIED ELECTRICITY 



^ 



^ 



a 



Fig. 260 



t 14 



¥1 



the joist and studding. When the wires are run in a direction 
perpendicular to the direction in which the joists run, holes are 
drilled in the joists and porcelain tubes, with a shoulder on one 
end, are driven into these holes, through which the wires are 
passed, as shown in Fig. 260, The knobs should support the 

wires at least 1 inch from the 
surface over which they run; 
these knobs should not be far- 
ther apart than 4^/^ feet and 
the wires fastened to them 
with tie wires having an 
insulation equal to that of the main conductor. Split knobs 
are often used for this class of work, which does away with 
the necessity of using tie wires. The various wires should 
he at least 5 inches apart at all times when supported on 
the knobs, and it is best to have the wires 
run on separate joist or studding when pos- 
sible, as shown in Fig. 261. Porcelain tubes 
should always be used where the wires pass 
through walls and ceilings, and the knobs 
should be so located that there is no strain 
on the tubes. Each wire must be encased 
in a piece of flexible tube at all outlets, 
switches, distributing centers, etc., and this 
piece of tubing should be of sufficient length 
to exten'd from the last insulator and project 
at least 1 inch beyond the outlet, as shown in 
Fig. 260. When wires cross pipes or other 
wires, they should be insulated by means of 
porcelain tubes or pieces of flexible tubing, as 
shown in Fig. 256. 

362. Interior Conduit and Armored Cable 
Work. — Armor cable consists of rubber-covered 
wire that is protected from mechanical injury 
by two layers of flexible steel armor, which 
also serves to protect the wire to a certain 
extent from dampness, Fig. 262. A lead covering 
is often placed between the insulation on the wire and the steel 
armor, which protects the enclosed conductors from dampness. 
Leaded armor cables can be used for all classes of work, and 
the unleaded cable can also be used for all classes where it 



y 



Fig. 261 






ELECTEIC WIEING 337 

is not exposed to dampness. Leaded cables may be buried in 
the walls or floors, but reasonable care must be exercised in 
installing the unleaded cable to protect it from being exposed 
to moisture. Special fittings are on the market for con- 
necting the ends of the armor to metal outlet boxes. The 
armor should be cut away 6 or 7 inches from the end 
of the wires so that switches, fixtures, etc., may be properly 
connected. 

Tn new work, holes 
are drilled in the joists 
and studding and the 
cable is pulled into 
place, it being fastened 
by means of special Fig. 262 

straps of metal. Ar- 
mored cable is used quite extensively in the wiring of old 
buildings, as it can be put into different places regardless of 
its coming in contact with pipes, etc. 

In conduit work approved rubber-covered wire must be 
used and it is drawn into a system of iron piping that is 
installed and connected to all outlet and cut-out boxes before 
the wires are put in place. The conduit must never have an 
inside diameter of less than .625 inch and in installing it no 
bends or elbows should have a radius less than 3.5 inches, 
nor should there be more than the equivalent of four quarter 
bends from outlet to outlet. The conduits should be*cleaned 
out before the wires are drawn in and it is often advisable 
to blow a small quantity of soapstone into them, which will 
reduce the friction of the wire on the surface of the pipe to 
a minimum. This system of wiring is the safest, most satis- 
factory, and in the long run no doubt the most economical 
method of installing wires. 

363. Service Wires. — When the electrical energy is sup- 
plied from a source outside the building, certain precautions 
must be taken in bringing the wires into the building. "When 
the wires are overhead and are taken into the building above 
the basement, they must be provided with drip loops and pass 
through insulating tubes in the wall that slope so their out- 
side end is lower than the inside end. Such an arrangement 
is shown in Fig. 263. When the wires are brought into the 
basement from an outside service that is overhead, they 



328 



PRACTICAL APPLIED ELECTRICITY 



should be placed in a metal conduit with its outside end bent 
over and provided with a suitable fitting, thus forming a 
drip loop, as shown in Fig. 264. When the outside wires are 
underground and are brought into the basement, they 
should be placed in metal or porcelain tubes that are tightly 
sealed afterwards. The leads pass directly to the cut-out, 
then to the main switch, both of which should be enclosed 
in a suitable cabinet, and from the switch to the meter, and 




Insulatino 
tube ^ 




Fig, 263 



Fig. 264 



then to the main distributing center. A three-branch dis- 
tributing center for a three-wire system is shown in Fig. 
265. The gaps in the various leads are for fuses. 

364. Location of Outlets, Switches, and Distributing Board. 
— The various distributing boards from which the branch cir- 
cuits emanate should always be located as near the center 
of the load as possible, so that the circuits will be practically 
the same length. These boards are mounted in either metal 
boxes. or wooden boxes with fireproof lining, depending upon 
the kind of work. Each branch circuit is provided with a 
double-pole switch and properly fused. A three-wire distrib- 
uting board is shown in Fig. 266 and a two-wire board in 
Fig. 267. A main switch and fuses are placed in the leads 
connecting the board to the source of energy. 

The switches in the various branch circuits outside the 



5l 



ELECTEIC WIKING 



329 



distributing board should be located in such positions that 
they will be convenient to operate. They should, as a rule, 
in light wiring be placed near a door and on the side nearest 
the door handle. 

The location of the outlets will depend upon the location of 
the lamps, kind of lamps — arc or incandescent — location of 
motors, etc. A chart of the standard symbols for wiring 
plans, as adopted and recommended by the National Electrical 
Contractors' Association of the United States, is given in 
Chapter 20. 



EM3 nz3. 



H H H , 



nm^ r jir 



MTrn 



Q fl T ^ M 



limE Ce; MO 



M~^ i^ioia 



uuu 



hTM mrns 




Fig. 265 



Fig. 266 



365. Installing Arc Lamps and Fixtures. — Arc lamps should 
be insulated from inflammable material, and at all times sur- 
rounded with a glass globe surrounding the arc and securely 
fastened in place. Approved rubber-covered wire should b€ 
used and it should be supported on porcelain or glass insu- 
lators in constant-current systems that hold the wire at least 
1 inch above the surface over which the wire passes, and 
these wires should never be nearer each other than 8 Inches, 
except in cut-out boxes and in the lamps. 

Fixtures, when supported from the gas piping or from any 
grounded metal work of the building, must be insulated from 
such metal by an approved insulating joint placed as neai 
the walls or ceiling as possible. The cross-section of an 
insulating joint is shown in Fig. 268. 

366. Electric Heaters. — All electric heaters must be pro 
tected by cut-out and controlled by indicating switches. These 



330 



PRACTICAL APPLIED ELECTRICITY 



switches should be double pole except when the device con- 
trolled does not require more than 660 watts. Heaters must 
always be in plain sight unless special permission be given 
by the Inspection Department having jurisdiction. Stationary 
heaters, such as radiators, ranges, plate warmers, etc., must 
be placed in safe locations, isolated from inflammable ma- 
terials, and always treated as sources of heat. 





Figr. 267 



Fig. 268 



367. Electric Generators and Motors. — ^All generators and 
motors should be located in dry places. They should neve: 
be installed in places where a hazardous process is carried 
on, in dusty places, or in places where they are exposed to 
inflammable gases or flying of combustible materials. 

Generators and motors should always be insulated from the 
surrounding floor on wooden bases. When it is impossible 
to insulate them for any reason, the Inspection Department 
having jurisdiction may permit the omission of the insulating 
frame, in which case the frame should be permanently and 
effectively grounded. 

368. Electrical Inspection. — No attempt has been made in 
this chapter to give all the details that must be observed in 
the proper installatiori of electrical conductors, it being left 
to the engineer in charge, who should have a copy of both 
the National Electric Code and its supplement, which contains 
a list of approved - fittings and materials. The Inspection 
Department under whose jurisdiction the work is being done 



ELECTEIC WIEING 331 

should be consulted freely, as it will often save time and 
trouble in cases where doubtful work has been suspected, and 
the inspector requires the floors to be taken up or the plaster 
knocked off in certain places to satisfy himself that the work 
was properly done. 



CHAPTER XVII 
"the alternating-current circuit 



369. Definition of an Alternating Electromotive Force op 
Current.— An alternating electromotive force or current is 
one that changes in value, and reverses in direction at certain 
regular intervals. Such an electromotive force would be 
induced in a loop of v^ire that was resolved in a magnetic 
field, as shown in Fig. 126. An alternating cilrrent would 
exist in a closed circuit connected to the terminals of such a 
coil by means of two brushes and slip-rings, as shown in 
Fig. 127. 

370. Hydraulic Analogy of 
an Alterating Current.— The 
alternating current in an 
electric circuit can be com- 
pared to the flow of water in 
rl'i^ a closed pipe connected to a 
J cylinder in which there is an 
oscillating piston operating, 
as shown in Fig. 269. If 
the arm (A) be moved at a 
constant angular velocity 
there will be a flow of water through the pipe similar to that 
represented by the curve in Fig. 270. The length of the hori- 
zontal line (AB) corresponding to the time of one complete 
revolution of the arm (A), and the length of the ordinate of 
the curve, or vertical distance from a point on the line (AB) 
to the curve, at any instant, represents the rate at which the 
water or liquid is passing a given cross-section of the pipe, 
or the current in the pipe. 

371. Cycle — Frequency — Alternation — Period — Synchronism 
— Phase Displacement. — When an alternating pressure or cur- 

332 




Fig. 269 



THE ALTERNATING-CUEEENT CIRCUIT 



333 



A 




90" 



180 

Fig. 270 




270 360 



rent has passed through a complete set of positive and nega- 
tive values, starting from any value and again returning to 
that value in the same direction, the pressure or current has 
completed v/hat is called a cycle. A complete cycle is shown 
by curve (E) in Fig. 271, which represents an alternating 
electromotive force. The 
e.m.f. is zero at (A), in- 
creases to a maximum posi- 
tive value at (C), decreases 
to zero at (D) and reverses 
in direction, increasing to a 
maximum negative value at 
(E), and then decreases to 
zero at (B), which completes 
the cycle, it passing through 
a similar set of values in the 
next equal interval of time. 

The number of cycles the pressure or current passes 
through in one second is called. the frequency. Thus a 60-cycle 
electromotive force or current would be one that parsed 
through a complete set of positive and negative values 60 
times per second. 

An alternation is half a 
cycle, and corresponds to a 
complete set of positive or 
negative values of e.m.f. or 
current. There will be just 
twice as many alternations 
in a given time as there are 
cycles, or a frequency of 60 
cycles would mean 120 al- 
ternations per second. 

The period of an e.m.f. or 
current is the time in sec- 
onds required to complete one cycle. Thus the period of a 
60-cycle e.m.f. or current would be %o of a second and of a 
25-cycle e.m.f. or current 1^5 of a second. 

Two electromotive forces or currents are said to be in 
synchronism when they have the same frequency. 

Any number of e.m.f.'s or currents are said to be in phase 
when they pass through corresponding values of their re- 



K 


-v£ 


E 


B 


A < 




: D 




^0 


1 



90 



180 
Fig. 271 



270 



360 



334 



PRACTICAL APPLIED ELECTRICITY 




360 



spective cycles at the same time. Thus the current (I), 
and the e.m.f. (E), Fig. 271, are in phase because they both 
pass through corresponding values of their cycles at the same 
time. 

Any number of e.m.f. 's or currents are said to be displaced 
in phase when they do not pass through corresponding values 

of their respective cycles at 
the same time Thus the 
current (I) and the e.m.f. 
(E), Fig. 272, are displaced 
in phase, and this phase dis- 
placement may be measured 
in degrees by determining 
the difference in the degrees 
represented by the two 
points where the curves 
cross the horizontal line 
(AB). The total length of the line (AB) corresponds to 360 
degrees, and if the distance between the two points where 
the curves (E) and (I) cross the horizontal line is Vs of the 
length of the line (AB), the e.m.f. and current represented by 
these two curves are displaced in phase by % of 360 degrees, 
or 45 degrees. The current lags the e.m.f., because the current 
curve IS shown as crossing the horizontal line <AB), as you 
pass along the line from left to right, after the e.m.f. curve. 
This phase displacement may be expressed in time as well 
as degrees. Thus the time interval between when the current 
and e.m.f. are zero, as shown in Fig. 272, is % of a period or 
(1 -f- 8 X frequency) of a second. The displacement in prac- 
tice is usually measured in degrees rather than time. 

372. Chemical and Heating Effects of an Alternating Cur- 
rent. — It was mentioned in Chapter VIII that the chemical 
effect of an alternating current was zero. This is due to the 
fact that the current exists in the circuit in one direction for 
the same time that it exists in the opposite direction and as a 
result of this reversal in direction there will be a chemical 
action taking place first in one direction and then in the 
opposite direction, and the resultant chemical action will be 
zero. 

The power expended in heating a conductor at any instant 
is equal to the product of the resistance of the conductor and 



THE ALTEKNATING-CURRENT CIRCUIT 



335 



the square of the current in the conductor. In an alternating- 
current circuit the current is constantly changing in value 
and, as a result, the power expended in heating the conductors 
is changing in value, it at any instant being equal to the 
product of the resistance and the square of the current at 
that particular instant. In order to obtain the total heating 
effect of an alternating current, it is necessary to add up all 
of the instantaneous heating effects. The heating effect of 
a current is independent of the direction of the current in the 
circuit. 

373. Sine Wave E.M.F. or Current. — If a simple loop of 
wire be revolved at a constant rate in a uniform magnetic 
field, there will be an induced e.m.f set up in the coil, which 
will reverse in direction and change in value as the coil 
rotates. The value of the induced e.m.f. will vary as the 
direction of motion of the coil changes with respect to the 
magnetic field, it being a maximum when the two sides of 
the loop move perpendicular to the magnetic field and zero 
when the two sides of the loop move parallel to the magnetic 




Fig. 273 



field. The e.m.f. induced in the coil for positions between 
those just mentioned will bear a definite relation to the 
maximum e.m.f., and this relation can be determined as fol- 
lows: The rate at which the two sides of the loop are mov- 
ing perpendicular to the field decreases in value from a ver- 
tical position and becomes zero for a horizontal position of 
the coil when the field is vertical, as shown in Fig. 273. The 
movement of the coil at any instant can be resolved into 
two parts, one parallel to the magnetic field and the -other 
perpendicular to the magnetic field. It is the part that is per- 



336 



PRACTICAL APPLIED ELECTRICITY 



pendicular to the magnetic field that results in an e.m.f. being 
induced in the coil, and this part is proportional to the sine of 
the angle the path of the two sides of the coil make with the 
magnetic field. The sine of this angle will vary as the pro- 
jection of the coil on the vertical plane. If the projection 
when the coil is in a perpendicular position be taken as rep- 
resenting the maximum e.m.f., the e.m.f. for other positions 
will correspond to the projection of the coil upon the vertical 
plane for those positions. Let a complete revolution be rep- 
resented by the horizontal line (AB), it corresponding to 360 

degrees, then the relation 
of the e.m.f.'s for various 
angular positions can be 
laid off on ordinates drawn 
vertically through points 
on the line (AB), which 
correspond to the angular 
displacement of the coil 
from a position perpendic- 
ular to the field. The values 
for the 45° positions are the 
only ones shown in the fig- 
ure; the remaining ones, 
however, are determined 
in the same way. Such a 
curve is called a sine 
curve, since its ordinates vary as the sine of the angle rep- 
resented by the point on the line (AB) through which the 
ordinate passes. An e.m.f. or current whose value varies as 
the ordinate of a sine curve is called a sine e.m.f. or current. 
The following calculations are all based on a sine curve. The 
e.m.f. and current curves met with in practice are very sel- 
dom sine curves, but approach a sine curve quite often. 

374. Maximum, Average, and Effective Values of E.M.F. 
and Current. — The maximum value of an alternating e.m.f. or 
current is the value represented by the ordinate of the e.m.f. 
or current curve having the greatest length. Thus in Fig. 
274 the maximum e.m.f. occurs at 90° and 270°, it being op- 
posite in direction for the two positions but having the same 
value. 

The average value of an alternating e.m.f. or current is 




^60 



THE ALTERNATING-CUKRENT CIECUIT 337 

equal to the average of all of the instantaneous e.m.f.'s or 
currents for a complete alternation, starting with zero value 
and returning to zero value. For a true sine wave the aver- 
age e.m.f. and current are always .636 times their maximum 
value. This relation is determined by finding the area of a 
positive or negative loop of the e.m.f. or current curve and 
dividing this area by the distance between the two points 
where the curve crosses the horizontal line. The rectangles, 
shown by the shaded portions in Fig. 274, have each the same 
area as one loop of the sine curve, and the altitude of this 
rectangle is .636 times the maximum ordinate of the sine 
curve. 

The effective value of an alternating current is numerically 
equal to a steady direct current that will produce the same 
heating effect in a given time as is produced by the changing 
alternating current. If a conductor has a resistance of (R) 
ohms and there is an alternating current in the conductor, 
the power expended in heating the conductor at any instant 
is equal to the value of the current at that instant squared, 
times the resistance in which the current exists. Adding up 
all of these instantaneous heating effects for a certain time 
gives the total heating effect. This resultant or total heating 
effect could be produced by a steady direct current as well 
as by an alternating current. Now the value of the steady 
direct current required to produce the same heating effect as 
is produced by the alternating current corresponds in value 
to the effective alternating current 

I2R =: average i2R, 
or 

12 = average 12 
and 



I = V average 12 

Note — Small letters are used to represent instantaneous 
values. 

For a sine wave the square root of the average of the in- 
stantaneous values squared is equal to .707 times the max- 
imum current. 

Average value = .636 maximum value 
Effective value = .707 maximum value 
Effective value = 1.11 average value 



338 



PRACTICAL APPLIED ELECTRICITY 



The effective value divided by the average value is equal to 
1.11, which is called the form factor of the wave. The above 
relations hold true for sine waves only. 



Fig. 275 



.^ 




Fig. 276 



375. Vector Representation of Alternating E.M.F/s and 
Currents. — A vector quantity is one having both direction and 
magnitude; it may be represented by a line, called a 
vector, drawn in a definite direction corresponding to the 
direction of the quantity it represents and having a length 
corresponding to the value of the vector quantity to a suitable 
scale. Thus a current of 10 amperes and an e.m.f. of 10 volts, 
displaced in phase by 30 degrees, would be represented as 
shown in Fig. 275. 

These vectors can be thought of ,as rotating, and one 
revolution corresponds to 360 degrees. The counter-clockwise 
direction of rotation is taken as positive. In representing 
alternating e.m.f.'s and current by vectors, the effective values 
are the ones usually used. 

376. Addition and Subtraction of Vectors. — Two vectors are 
added in the same way as two forces are added in determining 
the resultant force. The two vectors (Ej) and (E2), shown in 
Fig. 276, are added by completing the parallelogram, as shown 
by the dotted lines, and drawing the diagonal gives the re- 
sultant (E). Its direction is that indicated by the arrowhead. 

Two vectors are subtracted by reversing one and adding 
them. Thus if it is desired to know the value of (E^ — E2), 
shown in Fig. 276, the vector (E2) is reversed in direction and 
then added to the vector (E^); the direction of the vector 
(E), representing the difference, is shown in the figure by the 
arrow head. ' 

377. Factors Determining the Value of an Alternating Cur- 
rent. — The current in a circuit upon which there is a steady 



THE ALTEENATING-CURRENT CIRCUIT 



339 



direct voltage impressed is equal to the value of the impressed 
voltage in volts divided by the total resistance in ohms. In 
an alternating-current circuit upon which there is a constant 
effective pressure impressed, the value of the effective current 
is determined not by the resistance alone but by the combined 
effects of the resistance, inductance, and capacity, if they all 
be present in the circuit. If there is no inductance or capacity 
in the circuit, then the same law holds for the alternating- 
current circuit as is true for the direct-current circuit. The 
current in the alternating-current circuit will be equal to the 
impressed voltage divided by the resistance of the circuit 





Fig. 277 



Fig. 278 



when the effects of the inductance and capacity are exactly 
equal, they acting in opposition to each other, as will be 
explained later. 

378. E.M.F/s Required to Overcome Resistance. — The e.m.f. 
at any instant required to overcome the resistance of a circuit 
is equal to the product of the current at that instant and 
the resistance of the circuit, or (IR). If there be an alternat- 
ing current, as represented by the curve (I) in Fig. 278, in a 
circuit, the e.m.f. at any instant required to produce this cur- 
rent is equal to (IR) and varies directly as the current or 
passes through corresponding values at the same time. The 
current and e.m.f. will, as a result, be in phase, and the curve 
(Er) represents the impressed e.m.f. required to produce the 
current (I). 

379. Hydraulic Analogy of Inductance. — If the electric cur- 
rent in an alternating-current circuit be represented by a fluid 
that flows through a pipe, as shown in Fig. 279, due to the 



340 



PRACTICAL APPLIED ELECTRICITY 




Fig. 279 



alternating pressure that is created by the pump (P), then an 
inductance in such a circuit can be represented by a fluid 
motor (M) similar to that shown in the figure. When such 

a motor is connected in 
the circuit, considerable 
time will be required for 
the current of liquid to 
reach a maximum steady 
value if a constant pres- 
sure be applied to the cir- 
cuit by the pump, on ac- 
count of the inertia of the 
wheel (W) that is attached 
to the fluid motor. Alter 
the pressure has been ap- 
plied to the motor for some 
time, the motor will have 
reached a speed such that it offers practically no resistance 
to the flow of the liquid through it. If, however, there be a 
change in the pressure produced by the pump, there will be 
a tendency for the current of liquid to change in value, and 
the action of the fluid motor will always be such as to tend 
to prevent a change in the 
current — that is, if the cur- 
rent tends to increase in 
value due to an increase in 
pump pressure, the motor 
will oppose this ir^x-ease, 
and if the current tends to 
decrease in value, due to a 
decrease in pump pressure, 
the motor will tend to pre- 
vent this decrease. The ac- 
tion of the motor at all times 

is such as to tend to prevent any change in the value of the 
current in the circuit of which it is a part. 

The motor in the hydraulic problem just discussed corre- 
sponds in action to the inductance of an alternating-current 
circuit in which there is an alternating current. In the 
hydraulic problem, if an alternating pressure be applied to the 
motor instead of a direct pressure, its direction of rotation 




360 



THE ALTEENATING-CUREENT CIRCUIT 341 

will change twice per cycle of the impressed pressure. The 
direction of rotation, however, will not change at the same 
instant the pressure produced by the pump changes on ac- 
count of the inertia possessed by the moving parts of the 
fluid motor. The motor will continue to rotate in a given 
direction for some time after the pressure has been reversed 
in direction. The velocity of the paddle wheel of the motor 
determines the value of the current of liquid through it, and 
the direction of rotation of the paddle wheel determines the 
direction of the current. Since the velocity of the fluid motor 
is not a maximum when the pressure of the pump is a max- 
imum, it reaching a maximum velocity in a given direction 
after the pressure produced by the pump has reached its 
maximum value in the same direction, and the velocity of the 
motor is zero after the pressure of the pump is zero, the cur- 
rent of liquid in the circuit must lag the pressure. 

380. Phase Relation of E.M.F. to Overcome Inductance and 
the Current in an Alternating-Current Circuit. — Assume there 
is a circuit containing inductance alone, and that there 
is a current in this circuit represented by the curve (I), 
Fig. 280. The magnetic field created by the current at any 
time will depend upon its instantaneous value. Thus the 
magnetic field or lines of force associated with the current 
will be a maximum when the current is a maximum, will de- 
crease or increase in value with a decrease or increase in the 
value of the current, and will reverse in direction at the same 
time the current reverses in direction. Since the above rela- 
tion exists between the current in the circuit and the magnetic 
flux produced by the current, a second curve (*) may be 
drawn, as shown in Fig. 280, whose ordinate at any instant will 
represent to a suitable scale the magnetic flux at that instant. 
With a change in the magnetic flux associated with the cir- 
cuit, there will be an induced e.m.f. set up in the circuit, 
which at any instant is proportional to the rate at which the 
flux is changing with respect to the circuit, or the rate at 
which the conductor forming the circuit is cutting the lines of 
force. By investigating curve ($) it is seen that the flux is 
changing at its greatest rate when it is zero in value and is 
changing at a minimum rate when it is at its maximum value; 
or the induced e.m.f. in the circuit is zero at the points (C) 
and (E) and is a maximum at the points (A), (D), and (B). 



342 



PRACTICAL APPLIED ELECTRICITY 



The value of the induced e.m.f. at any other time during the 
cycle will depend upon the rate at which the flux is changing 
in value, and its direction will, according to Lenz's Law, al- 
ways be such as to oppose a change in the value of the current 
in the circuit. Thus, if the current be increasing in value in 
the positive direction, as it is between the points (A) and (C), 
the induced e.m.f. acts in such a direction as to oppose this 
change in the current, or it acts in the negative direction. If 
the current be decreasing in value in the positive direction, 
the induced e.m.f. opposes this decrease, or it acts in the 
positive direction. The curve (ei) represents the e.m.f. 
induced in the circuit. If the circuit has no resistance (a 
theoretical condition), the only e.m.f. required to produce the 
current (I) in the circuit would be one that would overcome 
the e.m.f. (ei). Such an e.m.f. would be represented by the 
curve (El), whose ordinates are at each instant equal to 
those of (ei) but opposite in sign, or (Ei) acts opposite to 
(ei), and will produce the current (I). From the relation 
of the curve (I) and (Ei) in the figure, it is seen that the 
current and impressed e.m.f. in an inductive circuit are dis- 
placed in phase by 90° and that the current lags the e.m.f. 

381. Hydraulic Analogy 
of Capacity. — The hydraul- 
ic analogy of a condenser 
is shown in Fig. 281. A 
flexible rubber diaphragm 
(D) is stretched across a 
specially constructed 
chamber (C) which is con- 
nected to a pump (P), as 
shown in the figure, that 
produces an alternating 
pressure. When the dia- 
phragm (D) is in its nor- 
mal position, it offers no opposition to the flow of the 
liquid through the pipe in either direction. As soon, how- 
ever, as the diaphragm is displaced from its neutral posi- 
tion, it sets up a reaction which opposes the flow of the 
liquid, and this reaction will increase in value as the dia- 
phragm is displaced more and more, finally reaching a value 
equal to the pressure, causing the liquid to flow through 




Fig. 281 



THE ALTEKNATING-CUREENT CIRCUIT 



343 



the pipe, and when the reaction becomes equal to the acting 
pressure there will be no current. If, now, the current be re- 
versed in direction, the diaphragm will return to its normal 
position, and while it is returning, ^^ will act in the same 
direction as the current. When the diaphragm has reached 
its normal position and it is forced to the opposite side, it 
will immediately react upon the current and tend to stop it, 
and this reaction will finally reach such a value that there 
will be no current in the circuit. It is seen, then, that the 
diaphragm acts with the current one-half of the time and in 
opposition to it one-half of the time. The reaction of the 
diaphragm corresponds to the electrical pressure at the 
terminals of a condenser connected in an alternating-current 
circuit, and it has a maximum value when the current is zero 
and a zero value when the current is a maximum. 

382. Phase Relation of the E.M.F. to Overcome the Effect 
of Capacity and the Current in an Alternating-Current Circuit. 
— The current in a circuit containing capacity alone may be 
represented by a curve such as (I), Fig. 282. The flow of 
liquid through the chamber (C), Fig. 281, could be repre- 
sented by such a curve, and 
since the reaction of the t 

diaphragm corresponds to 
the e.m.f. • at the terminals 
of the condenser, the phase 
relation of the e.m.f. and 
the current can be deter- 
mined by an investigation 
of Fig. 281, carrying the op- 
eration through a complete 

cycle. The position of the diaphragm must be normal 
when the current is a maximum, say at the point (C), 
Fig. 282, as it produces no opposition to the flow of 
liquid. As the diaphragm is extended to either side, its 
reaction increases and it opposes the flow of liquid, or if 
the current is in the positive direction, the action of the 
diaphragm is negative. If the current reverses in direction 
when it has reached a zero value and starts to increase in 
value in the negative direction, the diaphragm will act with 
the current or they will both be negative. When the di- 
aphragm has reached its normal position, its reaction is zero 




360 



344 PRACTICAL APPLIED ELECTRICITY 

and the current is a maximum, and at this point the reaction 
of the diaphragm changes sign and the current starts to 
decrease in value in the negative direction. While the cur- 
rent is decreasing in value in the negative direction, the reac- 
tion of the diaphragm is increasing in value in the positive 
direction. The current again reverses in direction after reach- 
ing zero value and starts to increase in the positive direction, 
the reaction of the diaphragm decreasing in value in the posi- 
tive direction, which completes the cycle. The curve (Oc), 
Fig. 282, represents the reaction of the diaphragm, or the 
electrical pressure at the terminals of a condenser connected 
in an alternating-current circuit carrying a current (I). Since 
the curve (ec) represents the reaction in the circuit, the e.m.f. 
required to overcome this reaction must be equal in value and 
opposite in direction to it at each instant. The curve (Ec) 
then represents the e.m.f. required to overcome the effect of 
the capacity in a circuit and produce the current (I). It is 
seen by an inspection of the curves that the current (I) leads 
the impressed e.m.f. (Ec) by 90 degrees. 

383. E.M.F. Required to Overcome Combined Effects of 
Resistance, Inductance, and Capacity. — By comparing the 
curves (Ei) and (Ec) in Figs. 280 and 282, it is seen that 
they are displaced in phase by 180 degrees, or the e.m.f.'s they 
represent act just opposite to each other. If now inductance 
and capacity be present in a circuit at the same time, the 
electrical pressures required to overcome their effects would 
tend' to neutralize. When the effects of inductance and 
capacity are equal, the two e.m.f.'s exactly neutralize, and the 
only e.m.f. required would be that to overcome the resistance 
of the circuit. The current and impressed e.m.f. in such a 
circuit would be in phase. If, however, the e.m.f.'s required 
to overcome the effects of inductance and capacity are not 
equal, they will not exactly neutralize and there will be a 
resultant e.m.f. in the circuit to overcome, which has a value 
at any instant equal to the difference between the e.m.f.'s to 
overcome the effects of inductance and capacity. The e.m.f.'s 
required to overcome the effects of resistance, inductance, and 
capacity, and thus produce the current (I), are shown in 
Fig. 283. The e.m.f. required to overcome the effect of in- 
ductance in this case is greater than that required to overcome 
the effect of capacity, and they will not exactly neutralize 



THE ALTERNATING-CURRENT CIRCUIT 



345 



each other. The curve (R) represents their combined value. 
This resultant pressure represented by the curve (R) must 
now be combined with the e.m.f. to overcome the resistance 
(Er) in order to obtain the impressed pressure required to 
produce the current (I). These two curves can be combined 
by adding their ordinates for 
various points along the hori- 
zontal line (AB), the algebraic 
sum of their ordinates at any 
point being the ordinate of 
the resultant curve (E), which 
represents the total pressure 
that must be impressed upon 
the circuit to produce the cur- 
rent (I). When the pressure 
required to overcome the ef- 
fect of the inductance iso' 
greater than the pressure re- 
quired to overcome the effect 

of the capacity, the current lags the impressed pressure, as 
shown in Fig. 283. If the pressure required to overcome the 
effect of the capacity is greater than that to overcome the 
effect of the inductance, the current in the circuit will lead 
the impressed pressure. 

384. Numerical Values of E.M.F. Required to Overcome 
Resistance, Inductance, and Capacity. — If the current in a cir- 
cuit changes in value according to the sine law, and (I) be 
taken as the effective value of the current in the circuit, the 
pressures required to overcome the effect of the inductance 
and capacity can be determined by means of the equations 
Ei=2X7rXfXLXl (131) 







D ~~^. 


A^ 


A y 


\ J 

\ 
\ 
\ 




^0 



90 180 

Fig. 283 



270^ 3&0 



Ec = ■ 



(132) 



2 X ttX f X C 

In the above equations (f ) represents the frequency in cycles 
per second, 7r = 3.1416, (L) is the inductance of the circuit in* 
henrys, (C) is the capacity in farads, and (I) is the effective 
current in amperes. (Ei) and (Be) represent the effective 
e.m.f.'s required to overcome the effects of the inductance 
and capacity, respectively. The e.m.f. required to overcome 
the effect of inductance leads the current by 90 degrees, and 



346 PRACTICAL APPLIED ELECTRICITY 

the e.m.f. required to overcome the effect of capacity lags the 
current by 90 degrees. The e.m.f. required to overcome the 
resistance is equal to (RI) and it is always in phase with the 
current. These three e.m.f.'s can be represented by three 
vectors, as shown in Fig. 284, the counter-clockwise direction 
of rotation being taken as positive. 

385. Total Electromotive Force Required to Produce a 

Given Alternating Current. — If the two quantities (2 X tt X f X 

L X I) and (I-j-2X7rXfXC), Fig. 284, are equal in value, 

the vectors representing them will be 

equal in length. The resultant of these 

2TTfLlk^->\ ^V^ ^^^ vectors then will be zero, and the 

30* \^' only e.m.f. required to produce the 

^^ current (I) is that to overcome the re- 



I 



2TTfC 



2i 



^^ ^ ^ sistance, which is equal to (RI). 



/ If, however, the e.m.f. required to 

^90* overcome the effect of the inductance 

is greater than that to overcome the 

capacity, the vector (Ei), Fig. 285, will 



Fig. .284 be greater in length than the vector 

(Ec). The resultant of these two 
e.m.f.'s will be equal to (Ei — Ec), and its direction will corre- 
spond to that of the larger vector, or (Ei). This resultant 
must now be combined with (E,) in order to obtain the total 
e.m.f. required to produce the current (I), which can be done 
graphically, as shown in the figure. The resultant (E) is 
equal to the diagonal of a parallelogram whose sides are 
(El — Ec) and (Er), and its value is equal to the square root of 
the sum of the squares of the two sides, or 



E= V E2+ (Et — Ec)2 (133) 

Substituting in the above equation the values of (Ei) and 
(Ec) as given in equations (131) and (132), and the value 
of (Er), which is equal to (RI), gives the equation 



E = 'J (RI)2+ ( 27rf LI \ 2 (134) 

^ ^ 27rfC ^ 

By taking (I) from under the radical sign, this equation can 
be changed to the form 



THE ALTEENATING-CUERENT CIRCUIT 



347 



B 



= l^RM-( 



2 TT f L - 



27rf C 



(135) 



E 



In- 



cise) 



\K^+ ( 



2 7rfL- 



2 TrfC 
The above equation gives the value of 
the current (I) in terms of the im- 
pressed electromotive force (E), the 
ohmic resistance of the circuit (R), 
the inductance (L) in henrys, the 
capacity (C) in farads, frequency (f) 
in cycles per second, and the constant 
(27r), which is equal to (2 X 3.1416) 
or 6.2832. 

386. Impedance and Reactance of a 
Circuit. — The impedance of a circuit in 
which there is an alternating current is 

the total opposition offered by the circuit to the flow of the 
electricity through it. The letter (Z) is usually used to rep- 
resent the impedance. It is, from equation (136), numerically 
equal to 




Fig. 285 



Z = 



^R2+ (^ 



2 7rf L- 



-c) 



(137) 



27rf 

The impedance of a circuit is composed of two factors, the 
resistance and the reactance. The reactance is the quantity 
which, when multiplied by the current, gives the component 
of the impressed e.m.f. that is at right angles to the current. 
The resistance multiplied by the current gives the component 
of the impressed e.m.f. in phase with the current. The re- 
actance, which is usually represented by the letter (X), is 
equal to 

X = 2 TT f L 



(138) 

27rf C 
The above value of (X) is composed of two factors, (27rfL) and 
(l-7-27rfC). The quantity (27rfL) is called the inductance re- 
actance and is represented by the symbol (Xi), while the 



348 



PRACTICAL APPLIED ELECTRICITY 



quantity (1 -^ 27rfc) is called the capacity reactance and is 
represented by the symbol (Xe). In general: 

Z = V R-' -h X2 (139) 

or 

Z = VR2 4- (Xi— Xc)2 (140) 

The inductance reactance (Xi) is considered as positive and 
the capacity reactance (Xc) as negative, when they are being 
combined. The impedance and the reactance of a circuit are 
measured in ohms just as the resistance is measured in ohms. 

387. Impedance Diagram. — Since 
the impedance of a circuit is equal 
to the square root of the sum of the 
X=(Xl"Xc^ squares of two quantities, as given 
in equation (139), a right-angle tri- 
angle can be drawn, as shown in 
Fig. 286 Fig. 286, its three sides representing 

•the resistance, reactance, and im- 
pedance. Such a figure is called an impedance diagram. 
388. Impedances in Series. — Any number of impedances in 
series can be added by adding their resistances and reactances, 
respectively, which gives the resistance and the reactance of 
the resultant impedance. Thus two impedances, (Z^) and 
(Zo) connected in series, may be added as shown in Fig. 287a. 
both reactances being positive. If one of the reactances be 
negative, the impedances are added as shown in Fig. 287b, 





R, 

Fig. 287 a 





\x, 


Xe 




v ^ 


(X.+Xa) 

V 



Fig. 287 b 



the resultant reactance being negative; it may, however, 
be positive, depending upon whether the negative react- 
ance is greater or less than the positive reactance. If the 
resultant reactance is positive, the current lags thee.m.f.; and 



THE ALTEENATING-CUKEENT CIECUIT 349 

if the resultant reactance is negative, the current leads the 
e.m.f. The impedance of a series circuit composed of a num- 
ber of impedances (Zi), (Z2),(Z3), etc., can be calculated by 
substituting in the following general equation 
Z = V (Ri + R2 + R3 + etc.)2 + (Xi + X2 + X3 + etc.)2 (14I) 

Example. — Calculate the total impedance of a circuit com- 
posed of two impedances in series having resistances of 10 and 
5 ohms and reactances of 15 and — 5 ohms, respectively. 

Solution. — The total resistance of the circuit is (10 + 5), or 
15 ohms and the resultant reactance is [15+ ( — 5)], or 10 
ohms. Combining the total resistance and the resultant 
reactance gives 



Z = V (15)2+ (10)2= 18.03— 

Ans. 18.03— ohms. 
389. Impedances in Parallel. — In calculating the combined 
impedance of a number of impedances in parallel, an equation 
is used similar to equation (141), but instead of using the 
separate resistances and reactances, the conductance and sus- 
ceptance of the circuit are used. 

The conductance of a circuit is the quantity by which the 
e.m.f. must be multiplied to give the component of the current 
parallel to the e.m.f. 

The susceptance of a circuit is the quantity by which the 
e.m.f. must be multiplied to give the component of the cur- 
rent perpendicular to the e.m.f. 

Admittance, conductance, and susceptance are all measured 
in a unit called the mho. 

The conductance is represented by the letter (G) and it is 
numerically equal to 

R R 

G-= -=— (142) 

R2 + X2 Z2 

The susceptance is represented by the letter (B) and it is 
numerically equal to, 

X X 

B = = — (143) 

R2 + X2 Z2 

The reciprocal of the impedance of a circuit is called the 
admittance and is represented by the letter (Y). 



350 PRACTICAL APPLIED ELECTRICITY 

1 

Y = — (144) 

Z 
Since 

E 
Z = ~ (145) 

I 
Then 

Y = I-f-E (146) 

The admittance of a circuit bears the same relation to the 
conductance and susceptance as exists between the impedance, 
resistance, and reactance, or 



Y = V G2 + B2 (147) 

The total impedance of a number of devices connected in 
parallel then is equal to the reciprocal of the admittance, 
which can be determined by the equation 



Y = V (Gi + G2 + etc.)2 + (Bi + B2 + etc.)2 (148) 

Example. — Calculate the total impedance of two impedances 
in parallel having resistances of 8 and 6 ohms, and reactances 
of 4 and 5 ohms, respectively. 

Solution. — Substituting in equation (142) gives the value of 
the conductance of the first branch (Gi), equal to 
8 8 1 

Gi = = — = — = .1 mho 

82 + 42 80 10 

and the conductance of the second branch (G2) equal to 
6 6 

G2 = = — =. .983 mho 

62 + 52 61 
Substituting in equation (143) gives the value of the suscept- 
ance of the first branch (Bi), equal to 
4 4 1 

Bi = = — = — == .05 mho 

82 + 42 80 20 

and the susceptance of the second branch (B2), equal to 
5 5 

B2 = = — = .0819 mho 

62 + 52 61 

The total conductance (G) of the circuit is equal to (Gi + G2), 



THE ALTERNATING-CUREENT CIRCUIT 



351 



G=.l + .0983 = .1983 
and the total susceptance (B) of the circuit is equal to 
(Bi + B2), or 

B = .05 + .0819 = .1319 
The admittance may now be obtained by substituting in equa- 
tion (147), which gives 



Y= V (.1983)2+ (.1319)2 



:V .039 323 + .017 398 



= V .056 72 
= .238 mho 
The impedance is equal to the reciprocal of the admittance, or 

1 

Z = = 4.20 

.238 

. Ans. 4.20 ohms. 

390. Phase Relation of the Current and Potential Drops in 
a Series Circuit. — The series circuit shown in Fig. 288 is com- 
posed of three parts, a resistance (R), an inductance (L), and 
a condenser (C). The current is the same in all parts of such 




Fig. 288 



a circuit and an ammeter, that will operate on alternating 
current, may be connected in the circuit at any point and it 
will indicate the current that exists in the circuit. 

A voltmeter (V) may be connected across the entire circuit, 
as shown in the figure, and it will indicate the drop in poten- 
tial over all three parts of the circuit combined. Three other 
voltmeters (Vr), (Vi), and (Vc), may be connected across 



352 



PRACTICAL APPLIED ELECTRICITY 



the resistance, inductance and capacity, respectively, as shown 
in the figure, and their indications will be a measure of the 
drop in potential over the three parts of the circuit. The sum 
of the indications of the three voltmeters, (Vr), (Vi), and 
(Vc), will be greater than the indications of the voltmeter 
(V) for the following reason: The drop in potential over the 
resistance is in phase with the current, the drop in potential 
over the inductance leads the current, and the drop in potential 
over the capacity lags the current. Since these three drops 
in potential are not in phase, their resultant is not equal to 
their numerical sum. The drops over the inductance and 
capacity may neutralize each other, being opposite in 
phase, in which case the voltmeter (Vr) would indicate the 
same drop as the voltmeter (V). In 
an alternating-current circuit the sum 
of the drops over the various parts of 
the circuit is not equal to the im- 
pressed voltage except in a circuit con- 
taining resistance alone. 

391. Phase Relation of Currents and 
Potential Drops in a Divided Circuit. — 
A divided circuit composed of three 
branches is shown in Fig. 289. The 
upper branch is a non-inductive resist- 
ance, the middle branch is an induct- 
ance, and the lower branch is a ca- 
pacity. A voltmeter (V) connected 
between the terminals (D) and (B) 
indicates the drop in potential over 
each of the three branches of the di- 
vided circuit. The current in any one 
of the branches is equal to the indica- 
tion of the voltmeter (V) divided by 
impedance of the branch. If the relation between 
reactance and the resistance is the same in each 
branch, the current in the various branches will be dis- 
placed in phase from the pressure indicated by the volt- 
meter (V), the same amount, or the several branch currehts 
(Ir), (Ii), and (Ic) will be in phase. The current (I) indi- 
cated on the ammeter (A) connected in the main line is the 
numerical sum of the branch currents, when they are all in 




Fig. 289 



the 

the 



THE ALTERNATING-CUBRENT CIRCUIT 



35'c 



f\ 


N 


^1 


e: b 


A ( 






A 

9 



. 90 



180 

Fig. 290 



2.70 



360 



phase, and the vector sum when the branch currents are not 
in phase. 

392. Instantaneous Power in an Alternating-Current Circuit. 

— The instantaneous power in a circuit at any time is equal to 
the product of the current 
and the e.m.f. at that par- 
ticular instant. The two 
curves (I) and (E), Fig. 
290, represent the current 
in a circuit and the e.m.f. 
acting on the circuit. These 
two curves are in phase 
and the power in the cir- 
cuit is represented by the 
curve (P), its ordinates 
being proportional to the 
product of the ordinate of 
the other two curves. This 
product is positive in sign 
at all times since the 

sign of both the current and the e.m.f. changes at the same 
time, and both loops of the curva (P) are drawn above the 
horizontal line. If the current and the e.m.f. be displaced in 

phase, as shown in Fig. 291, 
the product of their in- 
stantaneous values is not 
positive throifghout the 
cycle, but it is negative in 
sign fcr a portion of the 
time and, as a result, part 
of the curve (P) is below 
the horizontal line. The 
loops of the curve (P) above 
the horizontal line represent 
an output from the source 
of energy and the loops be- 
low the horizontal line rep- 
resent an input into the 
source of energy. The actual output then is proportional to 
the difference in area of an equal number of upper and lower 
loops. When the current and the e.m.f. are in phase, there 




360 



Fig. 291 



354 PRACTICALr APPLIED ELECTRICITY 



I 



are no lower loops, and the power in the circuit might be 
thought of as all being positive. If the current and the e.m.f. 
be displaced in phase by 90 degrees, the upper and lower 
loops are equal in area and the resultant power is zero. 

The current in any case may be divided into two parts, one 
part in phase with the e.m.f. and the other part making an 
angle of 90 degrees with the e.m.f., or at right angles to it. 
The power output due to the part of the total current at right 
angles to the e.m.f. is zero, because the area of the upper and 
the lower power loops are equal. The part of the total cur- 
rent in phase with the e.m.f. is all effective as far as power 
output is concerned, because the power loops will all be above 
the horizontal line.- 

When the current and the e.m.f. are in phase, the power is 
equal to the product of the effective e.m.f. and the current. 
If the current and the e.m.f. are displaced in phase, the power 
is not equal to the product of the effective e.m.f. and the 
current, but the current must be resolved into two parts, one 
part in phase with the e.m.f. and the other at right angles to 
the e.m.f. The part of the current or component in phase 
with the e.m.f. is equal to (I) times the cosine of the angle 
(9) between the current and the e.m.f. This component of 
the current times the e.m.f. gives the true power in the cir- 
cuit, or 

Power in watts = E I cos (149) 

in which (O) is the angle between the current and the e.m.f. 
and cos O is called the power factor. 

393. Determining the Value of the Power Factor. — Since 
the cosine of an angle such as (O), Fig. 128, is equal to the 
ratio of the line (c) to (b) or it is equal to (c^b), the power 
factor of a circuit can be easily determined when the con- 
stants of the circuit are known. The e.m.f. (E) and the cur- 
rent (I), Fig. 285, are displaced in phase by the angle (6). 
The line (HI) divided by the line (E) gives the cosine of (0), 
or the power factor. Since (E) is equal to (ZI), and Z = 
VR2 + X^ the value of the cosine of (9) may assume a 
Jtiumber of different forms. 

HI RI R R 

Power factor ^^ = = — = (150) 

E ZI Z V R2 4- X2 



THE ALTEENATING-CURRENT CIECUIT 355 

394. A Wattmeter Indicates the True Power in an Alter- 
nating-Current Circuit. — The indication of a wattmeter is pro- 
portional to the strength of two magnetic fields, one of which 
is produced by the load current and the other by the impressed 
pressure. The direction of these two fields must bear a dif- 
ferent relation to each other in order that the defiection of 
the moving system of the wattmeter be in the proper direc- 
tion. If one field reverses due to a reversal of the e.m.f. or 
current, the moving system of the wattmeter will tend to move 
over the scale in the opposite direction to what it did before 
the one field reversed in direction. If both fields reverse at 
the same time, the force tending to defiect the moving system 
does not change in direction. A wattmeter would indicate 
zero power if the force acting on the moving system acted for 
the same time in opposite directions. This would be the case 
when the e.m.f. and the current are displaced in phase by 90 
degrees, the upper and the lower power loops being equal in 
area. When the current and the e.m.f. are in or out of phase, 
the indication of the wattmeter is proportional to the average 
of all the values of the instantaneous power for a complete 
cycle, or the instrument measures the true power. When the 
e.m.f. and current are displaced in phase less than 90 degrees, 
the upper and the lower loops of the power curve are not 
equal. The force acting on the moving system corresponding 
to the lower loop is opposite to the force corresponding to the 
upper loop, and the resultant force is thus proportional to the 
difference in these two forces, which causes an indication 
corresponding to the true power. 

The product of the voltmeter and the ammeter reading, 
(E) and (I), does not give the true power in an alternating- 
current circuit unless the current and the e.m.f. are in phase. 
The product (E X I) is called the apparent power. In general, 
the wattmeter reading (P), or true power, equals (E X I), or 
apparent power multiplied by the power factor. 

P == E X I X power factor (151) 

Power factor == P -^- (E X I) (152) 



356 PRACTICAL APPLIED ELECTRICITY 

PROBLEMS ON THE ALTERNATING-CURRENT CIRCUIT 

1. A condenser of 132-microfarads capacity and an induct- 
ance of .061 henry are connected in series and a 60-cycle 
e.m.f. of 100 volts is impressed upon the circuit. Calculate 
the inductance and the capacity reactances, respectively. 

Ans. Inductance reactance (Xi) = 23.0 ohms. 
Capacity reactance (X,) = 20.0 ohms. 

2. If a resistance of 4 ohms be connected in series with 
the circuit given in problem 1, what will be the total Impe- 
dance of the circuit? What current will be produced by the 
impressed pressure of 100 volts? 

Ans. Impedance of entire circuit (Z) = 5 ohms. 

Current (I) = 20 amperes. 

3. Two impedances are connected in series and they each 
are composed of resistances of 10 and 12 ohms, and react- 
ances of —50 and 70 ohms. Calculate the total impedance of 
the circuit. 

Ans. 29.7 ohms. 

4. Calculate the admittance of a circuit having a suscept- 
ance and conductance of 3 and 5 mhos, respectively. What is 
the impedance of the circuit? 

Ans. Admittance (Y) = 5.83 mhos. 
Impedance (Z) = .171 ohm. 

5. The indication of a wattmeter connected to a certain 
load is 10 000 watts. An ammeter connected in the line indi- 
cates 125 amperes and a voltmeter connected across the load 
indicates 100 volts. What is the impedance of the circuit and 
the power factor? 

Ans. Impedance (Z) = .8 ohm. 

Power factor (PF) = .8 

6. The effective value of a sine alternating current is 10 
amperes, what are the average and maximum values? 

Ans. Average current ^^ 9.00 amperes. 
Maximum current = 14.14 amperes. 

7. A circuit having a resistance of 3 ohms and a resultant 
reactance of 4 ohms is connected to a 100-volt line. Determine, 



THE ALTEENATING-CURRENT CIRCUIT 357 

(a) the impedance of the circuit, (b) power factor, (c) cur- 
rent, (d) apparent power, (e) true power. 

(a) Impedance (Z) = 5 ohms 

! (b) Power factor = .6 

) Ans. (c) Current =20 amperes 

(d) Apparent power = 2000 watts 

(e) True power =1200 watts 



CHAPTEE XVIII 

ALTERNATING-CURRENT MACHINERY 

395. Alternators. — An alternator is a machine for convert- 
ing mechanical energy into electrical energy, which is delivered 
as an alternating current to a circuit connected to the ter- 
minals of the machine. The fundamental electrical principle 
upon which the alternator operates is the same as that of the 
direct-current generator, namely, electromagnetic induction. 
Alternators, like direct-current machines, consist of two prin- 
cipal parts, a magnetic field and an armature. The commu- 
tator of the direct-current machine is replaced in alternators 
by slip-rings which are connected to the terminals of the 
armature winding, and with which brushes make continuous 
contact and thus conduct the electricity to and from the 
armature winding. 

396. Types of Alternators. — Alternators may be divided 
into three types, depending upon the mechanical arrangement 
of the magnetic field and armature, viz, 

(A) Alternators with stationary fields and revolving arma- 
tures. 

(B) Alternators with stationary armatures and revolving 
fields. 

(C) Alternator-- with both armature and field stationary 
and using a rotatmg part called the inductor. Such alterna- 
tors are called inductor alternators. 

Small machines are usually of the revolving armature type, 
as the e.m.f. generated is usually comparatively low and the 
current the brushes must carry is small and no difficulty is 
experienced in properly collecting such a current. A direct- 
current generator can be converted into a revolving-armature 
alternator by placing two collector rings on one end of the 
armature and connecting these two rings to points in the 
armature winding that are 180 electrical degrees apart. Such 

358 






ALTEENATING-CUEEENT MACHINEEY 



359 



a connection for a two-pole, ring-wound armature is shown in 
Fig. 292. The commutator is not shown in this figure. 

The necessity of collecting the 
armature current is overcome by mak- 
ing the armature the stationary part 
and revolving the field poles, the cir- 
cuit of the field winding being con- 
nected to the source of excitation by 
means of collector rings and brushes. 
In such machines the armature wind- 
ing is placed in a laminated frame 
that surrounds the revolving field. A revolving-field alterna- 
tor is shown in Fig. 293. 

The construction of the inductor alternator is such that 
both the armature and the field are stationary. The reluctance 
of the magnetic circuit in this type of machine is changed in 




Fig. 292 




Fig. 293 

value by means of projecting arms, on a revolving mass of 
iron called the inductor. The path of the magnetic circuit is 
through the armature coils and as a result of the reluctance 



360 



PRACTICAL APPLIED ELECTRICITY 



of this path changing, due to the rotation of the inductor, 
there will be a varying magnetic flux through the armature 
winding, which will result in an induced electromotive force 
in the winding. This induced e.m.f. will be in one direction 
for an increasing flux through the armature coils, and in the 
opposite direction for a decreasing flux, resulting in an alter- 
nating e.m.f. 

Alternators may also be classified into the following groups: 

(A) Single-phase Alternators. 

(B) Polyphase Alternators. 

397. Single-phase and Two-phase Alternators. — A single- 
phase alternator is one that produces a single electromotive 
force, and a polyphase alternator is one that produces two or 
more electromotive forces, which may or may not produce 
currents in circuits that are electrically independent. The 
electromotive forces in a polyphase alternator are related to 
each other only by the element of time, or they are said to 



/^ 


\/\ 


K 






k 1 


L \ 


D \ 


L 


B 


o* 9 


Q li 


io* r 


\ 


3^ 



Ea^ 



L 



Fig. 294 



Fig. 295 



differ in phase. Thus in a two-phase alternator, there are two 
electromotive forces which are displaced in phase by 90 de- 
grees, or they are said to be in quadrature. The armature 
inductors in which these electromotive forces are induced 
may or may not form independent windings on the armature. 
When these windings are not independent, they must each be 
connected to two independent collector rings. The e.m.f.'s 
between these two sets of rings can be represented by two 
curves (En) and (Ei,), Fig. 294, which are displaced in 
phase by 90 degrees, or they may be represented by the two 



ALTERNATING-CUERENT MACHINEEY 



361 



vectors (Ea) and (Eb), Fig. 295, that are at right angles 
to each other. The arrow (R) in Fig. 295 represents the 
direction of rotation. The number ot collector rings on 
such a machine can, however, be reduced to three, when 
they are independent windings on the armature, by using one 
ring as a common connection for both armature windings. 
Such an arrangement would constitute a two-phase three-wire 
system and the one in the previous case would be a two-phase 
four-wire system. In the two-phase three-wire system, the 
current in the common lead is equal to the vector sum 
of the currents in the two outside leads. Fig. 296. When 
there is the same current in each outside lead and the same 
phase relation exists between the currents and their e.m.f/s, 
the system is said to be balanced. The current in the common 
lead is not zero for a balanced load, however, as shown in Fig. 
296, it being the vector sum of (L) and (lb), or it is equal 
to (IJ. 

398. Three-Phase Alternators. — 
If three single-phase windings be 
placed upon an armature core and 
displaced 120 electrical degrees 
from each other, the electromotive 
forces induced in these three wind- 
ings will be displaced in phase 120 
degrees, as shown by the curves 
(Ea), (E„), and (Ec), in Fig. 297, 
and by the vectors (A), (B) and 
(C) in Fig. 298. Each of these 
windings may be provided with two 
slip-rings, there being six in all, 
and connected to electrically inde- 
pendent circuits. Such a system 
would constitute a three-phase six- 
mre system. The three circuits connected to the differ- 
Bnt phases may be kept practically independent of each 
Dther by using lour leads and connecting three of the col- 
ector rings together, as shown in Fig. 299. The lead (4) 
serves as a common return to the other three. When the 
hree receiving circuits connected between the mains (1), (2), 
3), and (4) have the same resistance and reactance, the 
'.ystem is said to be balanced and the current in the three 




Fig. 296 



[ 



362 



PRACTICAL APPLIED ELECTRICITY 



circuits are equal and displaced in phase from their electro- 
motive forces by the same angle, the three currents are then 
120 degrees apart and their vector sum, which is the current 
in main (4), is zero. Hence, for a balance load, main (4) 
carries no current, and this lead can be dispensed with and 
only three collector rings need be used, the other three being 
connected together, forming what is called the neutral point. 




' 90*" ' 180*' '270*' 360* 
Fig. 297 




Fig. 298 



This arrangement of connections is called the *'Y" or "star" 
scheme of connecting the three windings. Three of the six 
rings can be dispensed with entirely, one terminal of each of 
the windings being connected to the neutral point inside the 

armature, as shown in the 

A 

r--^=h^wm^K=> — 



c 



symmetrical diagram in 

L_ Fig. 300. 

Another method of con- 
^ necting the three wind- 

ings of a three-phase ma- 

3 chine is shown in Fig. 301, 
which is called the "A" I 

4 (delta) or "mesh" scheme. 
399. Relation of E.M.F. 

and Current in "Y" and 
"A" Connected Armatures. — The currents in the mains (1), 
(2), and (3), Fig. 300, are the same as the currents in the 
three windings (A), (B), and (C). 

If the positive direction of the electromotive forces and cur- 
rents in the windings (A), (B), and (C) be taken in the direc- 
tion indicated by the arrows in Figs. 300 and 301, then the 



Fig. 290 



ALTEB^VATIXG-CURBEXT MACHINERY 363 

l^LTeZTZ" "'"\^'^ ^^^'^-> - F'^- 300 wil, be equal 
Ind rBr .T. ''«7r^ '''''^■'''' *" ^•'^•^'^ i^ bindings (A) 
and (B) at must be remembered that the direction of the 
arrows does not represent the actual direction of Le emf s 
and currents, but only the assumed positive direction ) The 

iT<^f\'T '^*"^^° '""^ *^-° ^- ^'^ in thrwSngs 11; 

and (B), which are displaced in phase bv 120 degrees, is oh. 





Fig. 300 



Fig. 301 



tained by reversing the direction of the vector (B) and adding 
It to the vector (A), giving the result (A-B) Fig 309 The 
e.m.f/s between leads (2) and (3). and (3) and a are ob 
tamed in a_similar manner. These resultant e.m.f.'s wUl be 
equal to V 3 times the e.m.f. in any one of the windings which 
can be shown by reference to Fig. 303. (E-.) represent, the 

in" /bT ^T'"''^' '""^ '-^'' ^^^'-^^-^ ^^- --^■- - -in' 
ofJ^l' ,t''T?^ (EO and (E.) are each equal to 2 units 
of^m.f., then (a) will represent 1 unit and (d) will represent 
\ 3 units of e.m.f_Two (d), which is equal to (A-B) will 
be equal to (2 V 3) units of e.m.f. when (E.) and (E.) are 
■equal to 2 units of e.m.f. Hence (A-B) is equal to the ^^ 
times the e.m.f. per winding 

las the em.f. ,n the winding (A), or the e.m.f. between leads 
,for the "A" connection is equal to the e.m.f. in t7e windtn! 
.connected to the two leads. The current in lead (1) 1" equal 

lU (Br Thesfr"^" '^'"^^'^ '""^ ^""-^-^^^^ '^ wlidi'gs (T 
and (B). These two currents are displaced in phase ion 

Mdm. ,1 10 (A). Th* ciirrMt In ihe other leads are obtaloed 



S64: 



PRACTICAL APPLIED ELECTRICITY 



in a similar manner and for a balance load the current in any- 
lead will be equal to V 3 times the current in any winding. 

400. Connecting Receiving Circuits to a Three-Phase System. 
— When the receiving circuits connected to the various phases 
of a three-phase system are dissimilar or take unequal current, 
resulting in an unbalanced load, four mains should be used, 





d— J<— d- 



Fig. 303 



as indicated in Fig. 299, each receiving circuit then takes cur- 
rent from one of the windings (A), (B), or (C) over the main 
(4) and one of the outside mains. It is always desirable, 
however, in the operation of alternators to keep the loads on 
the various phases as near equal as possible and in practice 
the different receiving circuits are so distributed between the 
three phases as to satisfy this condition as nearly as possible. 
If three single-phase motors, all of the same capacity and 
carrying the same load, be operated from the three phases of 
a three-phase system, the system will be balanced and no cur- 
rent will exist in the lead (4) when the connections are made 
as shown in Fig. 299. Three lighting circuits taking equal 
currents will result in the same condition. If, however, one 
of the motors or one of the lighting circuits be disconnected, 
there will then be a current in lead (4). 

When the output of the three-phase alternator is used to 
drive three-phase induction motors, synchronous motors, and 
synchronous converters, the currents in each of the three 
windings will be equal and will be displaced in phase from 
their respective e.m.f.'s by the same angle, or the system is 
balanced. For balanced load only three leads are required and 



I 



ALTEENATING-CUREENT MACHINERY 



365 



either the "Y" connection without the neutral, as shown in 
Fig. 300, or the **A'' connection, as shown in Fig. 301, may 
be employed. 

The current in each receiving circuit in Fig. 304 is equal to 
the current in the lead connected to it, and the e.m.f. over any 
one of the receiving circuits in Fig. 305 is equal to the e.m.f. 
between the leads connected to that particular receiving cir- 
cuit. The current in each receiving circuit in Fig. 305 is 
equal to the current in each lead divided by the V 3, and the 
e.m.f. over each receiving circuit in Fig. 304 is equal to the 
e.m.f. between leads divided by the V 3. 

401. Measurement of Power in Single-Phase System. — The 
connections for the measurement of power in a single-phase 





Source of 



Fig. 304 



Fig. 305 



system are identical to those for the measurement of power 
in a direct-current circuit. The connections of a Weston indi- 
cating wattmeter are given in Fig. 306. The alternator (A) 
is supplying energy to the lamps (L) as a load. 

402. IVIeasurement of Power in a Two-Phase System. — In a 
two-phase four-wire system, the power is measured in each of 
the two phases by separate wattmeters as though they were 
single-phase circuits and the total power is obtained by adding 
the two wattmeter readings. 

In a two-phase three-wire system, the power may be meas- 
ured by two wattmeters, they being connected as shown in Fig. 
307. When the connections are thus made, the upper watt- 
meter indicates the power in phase (A) and the lower watt- 
meter indicates the power in phase (B). The total power 



366 



PRACTICAL APPLIED ELECTRICITY 



at any instant is equal to the sum of the indications on the 
two wattmeters. 

A single wattmeter may be used to measure the power in a 
two-phase circuit by connecting its current coil in the common 
lead, as shown in Fig. 308, and then noting its indication first, 
when the pressure circuit is connected to one outside lead, 

A Current Coil 

^ — ^ 




Common 



Pressure Coil 



Pressure 



ss 



^QOQQQr 



Load 



Fig. 306 



Current Coil 
Fig. 307 



^ 



Common 



1 Current Coil 



5M 



Load 



as shown by the full line in the figure, and second, when the 
pressure circuit is connected to the other outside lead, as 
shown by the dotted line in the figure. If the load on the two 
phases remain constant while these two readings are being 
taken, the total power output of the two-phase machine will 
be equal to the sum of the two wattmeter indications. 

When the pressure circuit 
is connected as shown by 
the dotted line, the watt- 
meter will not indicate the 
power delivered by phase 
(A), nor will it indicate the 
power delivered by phase 
(B) when the pressure con- 
nection is made as shown 
by the full line, unless the 
current in each phase is in phase with its e.m.f. The rea- 
son for this can be shown by referring to Fig. 296, 
in which (Ea) and (Eb) represent the e.m.f.'s of phases 
(A) and (B), (la) and (lb) represent the currents in 
the two phases, and these currents are shown displaced 
in phase from their e.m.f. 's by the angles (O^) and (82). 
The current in the common lead, or in the current coil of the 
wattmeter, is represented by the vector (I„), which is the 



Pressure Coil 



55A 



Fig. 308 



THE ALTERNATING-CUERENT CIRCUIT 3^7 

y resultant of (la) and (lb). When the pressure coil is con- 
nected across phase (A), the wattmeter indication is equal to 
(Ea X In X sin 83). The product of (!„) and (sin 63) is equal 
to the component of (In) that is in phase with (Ea). The 
indication of the wattmeter when the pressure connection is 
across phase (B) is equal to (Eb X In X cos Gg). The product 
of (In) and (cos 0,) is equal to the component of (I^) that 
is in phase with (Eb). If the component of (In) in phase 
with one of the e.m.f.'s, say (Ea), is equal to the current in 
phase (A), then the wattmeter will indicate the power in 
phase (A) when the pressure connection is made as shown by 
the dotted line. The component of (In) in phase with (Ea) 
will be equal to (la) only when the currents in the two 
phases are in phase with their e.m.f.'s. 

403. Measurement of Power in a Three-Phase System. — The 
power in a three-phase six-wire system can be measured as 
though it were three single-phase circuits, which in reality 

it is. The total power out- 
I /-^nnnnnr K P^^ ^^ ^ machine then is 

000000^1 :^ equal to the sum of the 

wattmeter indications in the 
three phases. In a three- 



■^000000^ 



-^WWT^ 



C 



vnmw^ 




(-nfwnmur-^ J ^x phase four-wire system the 

L /\ power per phase can be de- 

J I termined by connecting a 



wattmeter in each of the 
Fig. 309 leads (1), (2), and (3), and 

connecting their pressure 
circuits between these leads and the neutral lead (4), Fig. 299. 
The total power output is equal to the sum of the three watt- 
meter indications. In a three-phase three-wire system, the 
power per phase can be determined by connecting three watt- 
meters as shown in Fig. 309. The total power output of the 
three phases is equal to the sum of the three wattmeter indi- 
cations. The above connection will apply equally well when 
the receiving circuit is connected "Y" or "A/' provided the 
series coils are connected in series with each load and the 
pressure coils across the loads. 

Two wattmeters may be used in measuring the power in a 
three-phase three-wire system by connecting them as shown 
in Fig. 310. 



368 



PRACTICAL APPLIED ELECTRICITY 



The sum of the two wattmeter readings is the total power 
delivered to the three receiving circuits. When the power 
factor is below .5, the reading of one wattmeter will be nega- 
tive and the total power is then equal to the algebraic sum or 
numerical difference of the two readings. 

All of the above connections apply to either balanced or 
unbalanced loads, making them applicable to any practical 
case. 

If the system is balanced, only one wattmeter need be used 
in Fig. 309, and its indication multiplied by three will give the 
total power, since on a balanced load each of the wattmeters 
will indicate the same. The power in any balanced three- 
phase three- wire system is equal to (V 3^ E X I X cosO), 
(E) being the e.m.f. between leads, (I) the current in each 
lead, and (cos 6) the power factor. 

The power factor of a balanced three-phase circuit can be 
determined from the two wattmeter readings, when they are 
connected as indicated in Fig. 310, as follows: 
_Pi-P2 
Tangent = V 3 (153) 

P1 + P2 
Substituting the values of (Pi), which is the larger reading 
and will always be positive, and (P2), which is the smaller 
reading and may be positive or negative in the above equation, 

gives the value of the tan- 



Current Coil 



Pressure Coil 



Pressure Coil 



-nmrns" 




gent of (0). The angle 
(0) can then be deter- 
mined from the table of 
trigonometric functions. 
Chapter 20, and the cosine 
of this angle is the power 
factor. 

404. The Synchronous 
Motor. — In the operation of 
an alternating-current gen- 
erator, a mechanical force must be applied to rotate the 
moving part of the machine, and when a given conductor 
on the armature is under a north pole of the field, the 
current in the conductor is in such a direction that the mag- 
netic field exerts a force on it which opposes the movement of 
the conductor or the rotation of the armature. If the conductor 



Current Coil 

Fig. 310 



ALTEENATING-CURRENT MACHINERY 369 

moves out of the field of a north pole and into the field 
of a south pole, the current will be opposite in direc- 
tion and, as a result, there is still a force acting on the con- 
ductor which opposes the rotation of the armature. If the 
current output of the generator increases, this opposing force 
increases and more mechanical power must be supplied to 
drive the machine. 

If the field of an alternator be excited by a direct current, 
and an alternating current from some external source be sent 
through its armature, which is revolved by an eigine or motor 
at such a speed that a given inductor in the winding passes 
from a certain position under a north pole to a corresponding 
position under the next north pole during the time of one 
cycle, the motion of the armature will be aided by the cur- 
rent when the direction of the current is such that the force 
exerted by the magnetic field upon the various conductors aids 
the motion. Since the current reverses in direction when 
the inductor passes from one pole to the adjacent pole, which 
is of opposite polarity, the force exerted by the magnetic field 
upon the conductor remains constant in direction. If the 
speed of the alternator has been adjusted so that the above 
conditions are fulfilled, the engine or motor may be discon- 
nected and the alternator will continue to revolve at a con- 
stant speed, which is determined by the frequency of the 
supplied current and the number of poles comprising the mag- 
netic field of the motor. When an alternator is operated in 
the above manner it may deliver power to some load by means 
of a belt or direct connection, and it is called a synchronous 
motor. , 

Synchronous motors are designed to operate on single-phase 
or polyphase circuits, their operation, however, is more satis- 
factory on polyphase circuits. Motors of this kind are called 
synchronous because they run in synchronism with the source 
of supply. Their speed is not necessarily the same as that of 
the generator driving them, it being the same only when the 
motor and the generator have the same number of poles. The 
speed of a synchronous motor can be determined by the use 
of the equation 

2 X f X 60 
S = -- (154) 



370 PRACTICAL APPLIED ELECTRICITY 

In the above equation (f) is the frequency of the impressed 
voltage, (p) is the number of poles on the motor, and (S) is 
the speed of the motor in revolutions per minute. 

405. Operation of a Synchronous Motor. — The synchronous 
motor does not behave in the same way as a direct-current 
motor. For example: If the field of a direct-current motor 
be v^^eakened, the motor will speed up in order ttot its counter- 
electromotive force may reach the proper value. When the 
field of a synchronous motor is weakened, there is no change 
in its speed, since the motor must run in synchronism with 
the impressed e.m.f. A change in load on a synchronous 
motor or a change in field strength will result in a change in 
the phase displacement of the armature current and the im- 
pressed voltage When the motor is operating without load, 
the field current can be adjusted to such a value that the cur- 
rent taken by the motor is very small and its counter e.m.f. 
is practically in opposition to the impressed voltage. An 
increase in field current will cause the armature current to 
lead the impressed voltage, and a decrease in field current will 
cause the armature current to lag the impressed voltage. If a 
load be placed on a synchronous motor, its armature will lag 
a small amount behind the alternator driving it, and this angle 
will increase with an increase in the load. When the arma- 
ture of the motor lags, the counter-electromotive force is no 
longer in opposition to the impressed e.m.f. and a current will 
be produced which is just sufficient to supply the required 
torque to enable the motor to carry its load. By increasing 
the load, the displacement of the armature will become sufii- 
ciently great to cause the motor to be thrown out of synchron- 
ism, or it will "break down" and stop. 

406. Starting Synchronous Motors. — A single-phase syn- 
chronous motor cannot be started from rest by sending an 
alternating current through its armature, the field being ex- 
cited by a direct current, because the current in the armature 
is rapidly reversing in direction and tends to cause the arma- 
ture to rotate first in one direction and then in the other direc- 
tion. Single-phase synchronous motors must always be 
brought up to full speed by some outside source of power, such 
as another motor or engine, before they can be connected to 
the source of electrical power. 

The polyphase synchronous motor may be started by con- 



THE ALTERNATING-CURRENT CIRCUIT 



371 




-C H 



h it' h 




necting its armature directly to the line, the field circuit of 
the machine being open. When the machine has reached 
synchronous speed, the field circuit can be closed and the load 
gradually thrown on. The motor is usually connected to its 
load by means of a friction clutch. This method of starting 
synchronous motors has the great disadvantage that the 
machine takes an exceedingly large lagging current and this 
causes an excessive drop in the supply lead 
and a general disturbance of the distributing 
system. The taking of a large current from the | 
line can be avoided by the use of an auto-starter 
or compensator. The operation of an auto- 
starter is described briefly in section (415). 

Very large synchronous motors are usually 
started by means of an induction motor that 
can be operated from the same line the syn-^ 
chronous motor is operated from, or by means 
of a small engine or direct-current motor, on 
account of the very small torque exerted by the 
armature when it is connected to the line. 

407. Synchronizing. — In order that an alter- 
nator or a synchronous motor may be connected 
to a line, it must be in synchronism. Synchronizing con- 
sists in adjusting the frequency, the phase relation of two 
e.m.f.'s and their magnitude so that they will coincide when 
connected together. The general practice in synchronizing is 
first to adjust the speed of the incoming machine until the 
frequency of its e.m.f. corresponds to the frequency of the 
line to which it is to be connected, then to adjust the field 
current until the machine and the line voltage are the same. 
Adjust the phase relation of the incoming machine until it is 
in phase with the line, which can be determined by means of 
lamps, a synchronoscope, or a voltmeter, and when they are 
in phase, they may be connected directly together. 

When lamps are used in synchronizing, they should be con- 
nected as shown in Fig. 311, one in each phase. The switch 
should be so arranged that it can be closed the instant the 
machines are in phase and synchronism, viz, when the lamps 
are dark. Two transformers may be used with their primaries 
connected across the same leads but on opposite sides of the 
paralleling switch. Their secondaries may be connected so 



Fig. 311 



372 



PRACTICAL APPLIED ELECTRICITY 




that their e.m.f.'s are in series or in opposition when the 
machines are in phase and synchronism. When they are in 
opposition and a lamp in circuit, the lamp will be dark when 
the machines are in phase and synchronism, and if they are 
connected in series the lamp will be light. 

The synchronoscope is an instrument that gives an indica- 
tion of the phase relation of two e.m.f.'s, and the one of 
higher frequency can be determined by noting the direction 
in which the pointer on the instrument rotates. 

' In synchronizing machines it is always best to close the 
main switch when the incoming machine is coming into 
proper phase rather than going out *of it, as the inertia of 
the armature in one case assists and in the other retards 
prompt synchronizing. 

408. The Transformer. — The 
transformer in its simplest form 
consists of two separate and elec- 
trically independent coils of wire 
that are wound upon a laminated 
iron core that is common to both 
of the windings. One of the 
coils is connected to some source 
of electrical energy which may 
be high or low voltage, and re- 
ceives an alternating current from it; and the other coil is 
connected to a load to which it delivers alternating current at 
a low or high voltage. The coil of the transformer that is 
connected to the source of energy is called the primary coil, 
and the one that is connected to the load is called the sec- 
ondary coil, Fig. 312. When the transformer delivers energy 
at a higher voltage than that impressed upon the primary 
coil, it is called a step-up transformer; when it delivers energy 
at a lower voltage than that impressed upon the primary coil, 
it is called a step-down transformer. 

409. Action of the Transformer without Load.— A trans- 
former is said to be operating on zero load when the sec- 
ondary circuit is open and it is, of course, delivering no cur- 
rent. When there is no current in the secondary winding, 
there is a very small current in the primary winding for the 
following reason: The current in the primary winding will 
cause an alternating magnetic field to be set up through 



Primary Secondary 

Fig. 312 



THE ALTERNATING-CURRENT CIRCUIT 373 

both the primary and the secondary windings, which induces 
an electromotive force in both of them. This induced e.m.f. is 
in the opposite direction to the e.m.f. impressed upon the 
primary winding and very nearly equal to it. It is only this 
difference in e.m.f. that is available for producing a current 
in the primary winding, and since this difference is small, 
there will be a small current in the primary winding when 
there is no load on the transformer. This current is called 
the no-load current of the transformer. The induced e.m.f. 
In the secondary coil is in phase with the e.m.f. induced in 
the primary, and it is in opposition to the impressed e.m.f. 
on the primary, or the primary and the secondary e.m.f.'s are 
displaced in phase by 180°. 

410. Action of the Transformer on Load. — If the sec- 
ondary coil of a transformer be connected to a receiving cir- 
cuit and a delivering current, the transformer is said to be 
loaded. Since the e.m.f. induced in the secondary coil is 
180° from the impressed e.m.f. on the primary coil, the cur- 
rent in the secondary coil will produce a magnetizing effect 
which tends to lessen that produced by the small current 
already in the primary coil and, as a result, the variations in 
the magnetic flux passing through both of the coils is de- 
creased, which results in a decrease in the induced e.m.f. 
in the two coils. This decrease in counter e.m.f. in the 
primary coil results in an increase in the difference between 
the impressed e.m.f. and the counter e.m.f., which results in 
an increase of current in the primary coil. If the load on the 
secondary coil be increased or decreased there will be a pro- 
portional increase or decrease of current in the primary coil. 

411. Ideal Electromotive Force and Current Relations in 
a Transformer. — Neglecting all losses in the transformer, the 
following relations will exist between the primary and the 
secondary e.m.f.'s and the primary and the secondary currents. 
Since it is assumed that the same magnetic flux passes 
through both the coils, the ratio of the induced e.m.f.'s in the 
two coils must be the same as the ratio between the number 
of turns in the two windings. The induced e.m.f. in the 
primary is equal to the impressed e.m.f. when all losses are 
neglected^ and then 



I 



374 PRACTICAL APPLIED ELECTRICITY 

Ep Np 

— = — (155) 
Es Ns 

Since the magnetizing action of the ampere-turns in the two 
coils are equal and opposite, neglecting all losses, we have 

NpIp = NsIs (156) - 

or 

Ip Ns 

— ==— (157) 
Is Np 

The ahove equation states that the primary an(J the second- 
ary currents are to each other inversely as the number of 
turns in the two coils. 

412. Actual Electromotive Force and Current Relations in 
a Transformer. — In the previous section the relation of the 
e.m.f.'s and the currents was based upon the assumption that 
there were no losses in the transformer, which is not the 
case in practice. The principal losses to consider in the 
practical operation of a transformer are: 

(A) Loss due to no-load current. 

(B) The I2R loss in the primary and the secondary coils. 

(C) Loss due to magnetic leakage. 

The part of the no-load current in phase with the impressed 
e.m.f. on the primary coil represents a loss, and it is the no- 
load current that affects the ideal relation between the pri- 
mary and the secondary currents, as given in equation (157). 
The resistance of the primary and the secondary coils and 
magnetic leakage are, on the other hand, the only things that 
affect to any extent the ideal relation between the primary 
and the secondary voltages. If all of the magnetic flux created 
by the current in either the primary or the secondary coils 
passed through the other coil, there would be no magnetic 
leakage, which would correspond to an ideal magnetic circuit. 
Magnetic leakage in its effect is equivalent to an outside in- 
ductance that is connected in series with the coils of the 
transformer. If this inductance be represented by (L), then 
the e.m.f. required to overcome it when there is a current of 
(I) amperes in the circuit is equal to (27rfLI). The effect of 
coil resistance and magnetic leakage upon the ideal relation 
between the primary and the secondary voltages is shown by 
means of a vector diagram. 



THE ALTERNATING-CURRENT CIRCUIT 



375 



413. Vector Diagram of a Transformer. — The magnetic 
flux that passes through both the primary and the secondary 
coils is represented by the vector (*), as shown in Fig. 313, 
and the no-load current by the vector (L). The e.m.f. induced 
in the primary and the secondary coils will lag the magnetic 
fluk ($) 90°. 

The e.m.f. impressed upon the 
primary coil is used in overcom- 
ing the resistance of the coil, the 
counter e.m.f. induced in the coil 
by the flux ($), which passes 
through both coils, and the effect 
of magnetic leakage. The e.m.f. 
to overcome the resistance is in 
phase with the primary current 
(Ip), as shown in the figure, the 
vector (Ep) represents the e.m.f. 
required to overcome the e.m.f. 
induced in the primary coil by 
the flux (*), and the vector 
(27rfLpIp) represents the e.mf. 
required to overcome the effect 
of magnetic leakage in the pri- 
mary, which is 90 degrees in 
advance of the primary current. 
The vector (Ep) represents the 

voiitage impressed upon the primary coil. The voltage induced in 
the secondary winding is represented by the vector (Es), which 
bears the same relation to the e.m.f. induced in the primary 
coil as exists between the primary and the secondary turns, 
which has been assumed unity in this case. This vector will 
represent the voltage at the terminals of the secondary coil 
when there is no load on the transformer. When the sec- 
ondary coil is supplying a current, the terminal voltage drops 
on account of the (IsRs) drop and magnetic leakage. These 
drops must be subtracted from the total e.m.f. induced in 
the secondary coil, which gives the terminal voltage equal to 
(Es). The drop (RsIs) is parallel to (Is), and the drop 
(27rfLsIs) is perpendicular to (Is). If the secondary coil is 
supplying a current (Is) there will be a current in the primary 



2TTfL 




Fig. 313 



376 



PRACTICAL APPLIED ELECTRICITY 



1 



coil, which combines with the no-load current (lo), giving the 
true primary current (I). 

414. Types of Transformers. — Transformers may be 
divided into two types, depending upon the arrangement of 
the coils and magnetic circuit, viz, 

(A) Core-type transformers. 

(B) Shell-type transformers. 

In the core-type transformer the coils are placed outside of 
the magnetic circuit, as shown in Fig. 314. In the shell-type 
transformer the magnetic circuit surrounds the coils, as 
shown in Fig. 315. 

Transformers may be divided into two types, depending 
upon the kind of circuit they are to be used on, viz, 

(A) Single-phase transformers. 

(B) Polyphase transformers. 









y 


1^ 




/^ 


N 










V 


.y 




^ 


^ 













^\ !^ 






^ L^ 


1 



Fig. 314 



Fig. 315 



In a single-phase transformer there is only one set of pri- 
mary and secondary terminals, and the fluxes in the one or 
more magnetic circuits are all in phase. In the polyphase 
transformer there are a number of different sets of primary 
and secondary connections. These various sets can be used in 
the different phases of a polyphase system. In such a trans- 
former there are two or more magnetic circuits through the 
core, and the fluxes in the various circuits are displaced in 
phase. It is not necessary to always use polyphase trans- 
formers in polyphase circuits, as a number of single-phase 
transformers may be used, one in each phase. 

Transformers may be divided into three types, depending 
upon the nature of their output, viz. 



THE ALTERNATING-CURRENT CIRCUIT 377 

(A) Constant-potential transformers. 

(B) Constant-current transformers. 

(C) Current transformers. 

The constant-potential transformer is one so constructed 
that the relation of the primary and the secondary voltage 

. remains practically constant, regardless of the load on the 

^ transformer. 

The constant-current transformer is one so constructed that 
the secondary current remains constant in value, and when 

f the load on the transformer changes, there is a change in the 
e.m.f. induced in the secondary winding. 

The current transformer is one so constructed that the 

I secondary current always bears a definite relation to the pri- 
mary current. These transformers are used principally in 
connection with instruments where it is desired to send a 
definite fractional part of the total line current through the 
instrument. They correspond to the shunt in the direct- 
current circuit. 

415. The Auto-Transformer. — The auto-transformer con- 
f sists of an ordinary transformer with its primary and sec- 
ondary coils so connected with respect to each other that 
the e.m.f. induced in the secondary coils either aids or 
opposes the e.m.f. impressed upon the primary coil. When 
the e.m.f.'s of the two windings act in the same direction, 
it is said to be an auto-step-up transformation, and when 
they act in opposite directions it is said to be an auto-step- 
down transformation. 

416. Methods of Cooling Transformers. — In small-capacity 
transformers the radiating surface is ample to prevent an 
excessive temperature rise when the transformer is in opera- 
tion. With an increase in capacity of the transformer, there 
is a proportional increase in the energy loss, and hence the 
heat generated, but the radiating surface does not increase at 
the same rate and, as a result, the temperature rise will, as 
a rule, be greater in large transformers than it is in small 
ones, unless some means be provided for cooling them. 
Transformers may be classified according to the method em- 
ployed in cooling them as follows: 

• 

(A) Dry-transformers, self-cooling. 

(B) Oil-filled transformers, self-cooling. 



378 



PRACTICAL APPLIED ELECTRICITY 



(C) Transformers cooled by a blast of air. 

(D) Transformers cooled by a current of water. 

(E) Transformers cooled by a combination of the above. 

No special means is ever employed for cooling small trans- 
formers, as they are dry. 

Large transformers are cooled by filling the space sur- 
rounding the coils and core with a good quality of oil, which 
tends to equalize the temperature of the various parts and 
to conduct the heat to the containing c^se of the transformer, 
where it is radiated. The containing case is often corrugated 
or made with protruding ribs, which adds to its radiating sur- 
face, as shown in Fig. 316, 





Fig. 316 



Fig. 317 



Large transformers are often constructed so that they can 
be cooled by forcing a blast of air through them. The coils 
in such transformers are usually sprBad apart so as to form 
ducts through which the air may circulate. 

Large transformers are often cooled by circulating water 
through a pipe which surrounds the coils and core. This 
method of cooling, however, is usually used in combination 
with the oil-cooled type. 

417. Rotating Magnetic Field. — Suppose the projecting 
arms or poles on the field frame of a dynamo be divided into 



THE ALTERNATING-CURRENT CIRCUIT 379 

three groups, and the poles belonging to one group marked 
(Ai), (A2), (A3), etc., those of another group marked (Bi), 
(B2), (Bg), etc., and the third group (Ci), (C2), (C3), etc.] 
as shown in Pig. 317. If a winding be placed on the 'poles 
belonging to any group, alternate ones being wound in op- 
posite directions, and each of these windings then connected 
to a source of direct e.m.f., the inner end of the poles will 
be magnetized alternately north and south. If an alternating 
e.m.f. be used, the polarity of the poles belonging to any 
group will be reversed twice per cycle. By connecting the 
three groups of windings to the different phases of a three- 
phase circuit, any three poles that occur in succession 
around the frame will not be magnetized to a maximum 
polarity of the same time. The time required for the maximum 
polarity to pass from one pole to the next is one-third of a 
half cycle or one-sixth of a cycle. The maximum polarity is 
passed from one pole to the next around the frame, which 
results in what is termed a rotating magnetic field. The speed 
at which this field rotates can be determined as follows: Let 
(f) represent the frequency of the impressed voltage, (p) 
the number of poles per phase, and since two poles corre- 
spond to one cycle, the time per revolution of the field will 
be equal to 

2 p 

time per revolution = (158) 

f 
and 

f 
number of revolutions per second = (159) 

2 p 
418. Induction Motor.— If a hollow metal cylinder be 
mounted inside of a rotating field, there will be an e.m.f. in- 
duced in it, due to the relative motion of the field and the 
cylinder, and this e.m.f. will produce a current in the cylinder 
which reacts upon the magnetic field and causes the cylinder 
to. rotate. The path taken by the induced current 'is not 
vrery well defined in the case of a cylinder and, as a result it 
mil not all be useful in producing a tangential force. This 
iifliculty is overcome by slotting the cylinder in a direction 
Darallel to the axis about which it rotates. The flux passing 
Detween poles of opposite polarity can be greatly increased 



380 



PRACTICAL APPLIED ELECTRICITY 



4 



with the same current in the winding, by mounting the cyl- 
inder upon an iron core, which should be laminated. This 
increase in flux will result in a greater e.m.f. being induced 
in the cylinder and a greater current would result, which 
would increase the force tending to turn the cylinder. The 
above principles are those upon which the induction motor 
operates. 




Fig. 318 



The winding of an induction motor that is stationary is 
called the stator, and the moving part is called the rotor. 
The stator windings are usually placed in slots cut in what is 
called the stator core, instead of being wound upon poles as 
shown in Fig. 317. The stator core of a small induction motor 
is shown in Fig. 318. The rotor in its simplest form consists 
of copper conductors imbedded in slots in a laminated iron 
core. These conductors are all connected in parallel by cop- 
per collars, one being placed at each end. With this arrange- 
ment the current due to the induced e.m.f. passes in a direc- 
tion parallel to the axis about which the rotor rotates, and 
its effect in producing rotation is a maximum. This simple 
form of rotor is called the "squirrel-cage" type, Fig. 319. The 
inductors may or may not be insulated from the core. 



THE ALTERNATING-CURRENT CIRCUIT 381 

In some cases the rotor is provided with a regular wind- 
ing, and this winding is connected to an external circuit by 
means of slip-rings. 

419. Operation of the Induction Motor. — If the stator be 
connected to a source of energy and the rotor is free to turn, 
it will run at such a speed that the induced e.m.f. in the 
rotor winding will produce the current required to drive the 
rotor. The induced e.m.f. in the rotor depends upon the rela- 
tive movement of the magnetic field and the rotor winding. 




Fig. 319 



If the rotor were to run at the same speed that the magnetic 
field revolves, there would be no e.m.f. induced in its winding. 
Then in order that there be an e.m.f. induced in its winding, 
its speed must be less (in the case of a motor) than that of 
the magnetic field. If (Sf) represents the speed of the field, 
(Sp) the speed of the rotor, then the difference in speed of 
the rotor and the magnetic field is (Sf — Sp). This difference 
in speed divided by (Sp) is called the slip, and it is usually 
expressed as a per cent of the synchronous speed. When the 
motor is loaded, the rotor speed decreases in order that the 
current may increase in value, it depending upon the induced 
e.m.f., which in turn depends upon the ratio of the speeds of 
the rotor and the magnetic field. A three-phase induction 
motor is shown in Fig. 320. 

420. Speed Regulation of the Induction Motor.-^The speed 
of induction motors may be regulated by changing the value of 
the impressed voltage upon the stator, by changing the con- 
nections of the stator winding so as to change the number 
of poles, or by changing the resistance of the rotor winding. 



382 



PRACTICAL APPLIED ELECTRICITY 



With a decrease in impressed voltage, the stator flux is 
lessened and the rotor current decreases if the speed re- 
mains constant. If the torque the motor is to generate is to 
remain constant, it being proportional to the product of the 
flux and the rotor current, there must be an increase in rotor 
current on account of the decrease in stator flux, in order 
that the motor carry its load. The required increase in rotor 
current is produced by a decrease in rotor speed. 

If the stator winding be changed, so as to change the num- 
ber of poles, there will be a corresponding change in rotor 
speed, the slip remaining constant. 




If the resistance of the rotor be increased, the e.m.f. re 
quired to produce a given rotor current must increase, am 
in order that there be an increase in rotor e.m.f. there mus 
be a decrease in rotor speed. 

421. Methods of Starting the Induction Motor. — Polyphase 
induction motors may be started by connecting their statoi 



THE ALTERNATING-CURRENT CIRCUIT 



383 



windings directly to the line. The current taken from the 
line, however, is excessive, and an auto-starter or com- 
pensator is usually used. 

Single-phase induction motors will not start when their 
stator windings are connected to a single-phase circuit, unless 
special provision is made for starting. There are four meth- 
ods that are employed for starting single-phase motors, viz, 

(A) Hand starting. 
• (B) Split-phase starting. 

(0) Repulsion motor starting. 

(D) "Shading-coil" starting. 




Fig. 321 

(A) Very small induction motors may be started by giv- 
ing them a good start by hand. 

(B) In split-phase starting there are two circuits through 
the motor, and there is a phase difference between the cur- 
rents in the two branches, if the ratio between the resistance 
and reactance of the branches is different. This difference 
in phase of the currents in the two circuits produces the 
required rotating iield to start the motor. The starting 



J 



384 PRACTICAL APPLIED ELECTRICITY 

torque, however, is very small when the current does 
exceed full-load current. 

(C) If the field magnets of an ordinary direct-current 
dynamo be laminated and excited by an alternating current, 
there would be induced e.m.f.'s set up in the armature wind- 
ing, provided the brushes were changed from their original 
position. These induced e.m.f.'s would produce currents 
which would react upon the alternating magnetic field and 
produce a torque tending to cause rotation. A single-phase 
alternating-current motor constructed to operate in the above 
manner is called a repulsion motor. After the motor is up to 
speed the brushes are automatically disconnected and it 
operates as an induction motor. 

(D) The "shading-coil" consists of a single turn of copper 
a;bout a part of each field-pole. The flux through the part 
of the field-pole enclosed by this turn of wire does not 
change in value as rapidly as the fiux in the remaining por- 
tion of the pole, due to the magnetic effect of the current in 
the copper coil that is produced by the e.m.f. induced in it. 
As a result of the above condition the fiux travels across the 
pole-face and there will be a torque exerted upon the rotor. 

422. Induction Generator. — An induction motor when 
operating without load takes a very small current from the 
supply leads and the speed of its rotor is very near that of 
the magnetic field. If the rotor be connected to some source 
of power and speeded up to the same speed as that of the 
magnetic field, the electrical power intake of the stator will 
be very small, it being equal to the iron loss in the stator. 
By increasing the speed of the rotor until it is above syn- 
chronism, the stator will deliver power to the alternating- 
current leads, provided the alternating-current generator re- 
mains connected to the leads to fix the frequency. When an 
induction motor is so used it is called an induction generator. 

423. Frequency Changer. — An induction motor provided 
with a rotor having a winding with terminals connected to 
collector rings may be used as a frequency changer, that is, 
it may be used to change the frequency. When the rotor of 
the motor is held stationary, the magnetic fiux of the stator 
induces e.m.f.'s in the rotor winding that are of the same 
frequency as the alternating e.m.f.'s applied to the stator. 
If the rotor is run at one-half speed, in the direction the mag- 



THE ALTEENATING-CUEEENT CIRCUIT 385 

netic field rotates, the e.m.f.'s induced in the rotor windings 
will be one-half full frequency. By driving the rotor in the 
opposite direction to the direction in which the magnetic 
field revolves, the frequency is raised. Thus, if the rotor be 
revolved backward at one-half speed, the induced e.m.f.'s in 
the rotor windings will be one and one-half times the fre- 
quency of the e.m.f.'s impressed upon the stator windings. 

424. Synchronous Converter. — The synchronous converter 
is a machine for converting alternating-current to direct-cur- 
rent, or vice versa, or it may be used as a double-current gen- 
erator. The synchronous converter resembles a direct-cur- 
rent generator in general appearance, the chief difference 
being the addition of a number of collector rings at one end 
of the armature, and the use of a larger commutator and 
smaller magnetic circuit than is ordinarily used in a direct- 
current generator. 

When such a machine is driven by an engine or motor, it is 
capable of supplying either direct or alternating current, or 
both at the same time. It may be driven as a synchronous 
motor from an alternating-current source of energy and de- 
liver direct current, or it may be driven from a direct-current 
source of energy and deliver alternating current. 

The synchronous converter may be started as a synchronous 
motor, as described in section (406), or it may be started from 
the direct-current end. In starting from the direct-current 
end, the machine is brought up to a speed a little above syn- 
chronous speed and then disconnected from the direct-current 
source, and when the speed has decreased to synchronous 
speed the alternating-current end is connected to the alternat- 
ing-current leads. A synchronous converter is shown in 
Pig. 321. 



CHAPTEE XIX 



RESUSCITATION FROM APPARENT DEATH FROM 
ELECTRIC SHOCK 

By Augustin H. Goelet, M. D. 

Supplement to Electrical World and Engineer, September 6, 1902. 

425. Resuscitation. — The urgent necessity for prompt and 
persistent efforts at resuscitation of victims of accidental 
shocks by electricity is very well emphasized by the success- 
ful results in the instances recorded. In order that the task 
may not be undertaken in a half-hearted manner, it must be 
appreciated that accidental shocks seldom result in absolute 
death unless the victim is left unaided too long, or efforts at 
resuscitation are stopped too early. 

In the -majority of instances the shock is only sufficient to 
suspend animation temporarily, owing to the momentary and 
imperfect contact of the conductors, and also on account of 
the resistance of the body submitted to the influence of the 
current. It must be appreciated also that the body under 
the conditions of accidental shocks seldom receives the full 
force of the current in the circuit, but only a shunt current, 
which may represent a very insignificant part of the whole. 

When an accident occurs the following rules should be 
promptly executed with care and deliberation: 

426. Rule (1) — Remove the body at once from the circuit by 
breaking contact with the conductors. This may be ac- 
complished by using a dry stick of wood, which is a non- 
conductor, to roll the body over to one side, or to brush aside 
a wire, if that is conveying the current. When a stick is not 
at hand, any dry piece of clothing may be utilized to protect 
the hand in seizing the body of the victim, unless rubber 
gloves are convenient. If the body is in contact with the 
earth, the coat-tails of the victim, or any loose or detached 

386 



RESUSCITATION 



887 



piece of clothing may be seized with impunity to draw it 
away from the conductor. When this has been accomplished, 
observe Rule (2). The object to be attained is to make the 
subject breathe, and if this can be accomplished and con- 
tinued he can be saved. 

427. Rule (2) — Turn the body upon the back, loosen the 
collar and clothing about the neck, roll up a coat and place it 




Fig. 322 



under the shoulders, so as to throw the head back, and then 
make efforts to establish respiration (in other words, make 
him breathe), just as would be done in case of drowning. To 
accomplish this, kneel at the subject's head, facing him as 
shown in Pig. 322, and, seizing both arms, draw them forcibly 
to their full length over the head, so as to bring them almost 
together above it, and hold them there for two or three sec- 
onds only. (This is to expand the chest and favor the en- 
trance of air into the lungs.) Then carry the arms down to 
the sides and front of the chest, firmly compressing the chest 
walls, and expel the air from the lungs, as shown in Fig. 323. 
Repeat this manoeuvre at least sixteen times per minute. 
These efforts should be continued unremittingly for at least 
an hour, or until natural respiration is established. 

428. Rule (3) — At the same time that this is being done, 
someone should grasp the tongue of the subject with a 
handkerchief or piece of cloth, to prevent it slipping, and 



388 PRACTICAL APPLIED ELECTRICITY 

draw it forcibly out when the arms are extended above the 
head and allow it to recede when the chest is compressed. 
This manoeuver should likewise be repeated at least sixteen 
times per minute. This serves the double purpose of free- 
ing the throat so as to permit air to enter the lungs, and 
also, by exciting a reflex irritation from forcible contact of 
the under part of the tongue against the lower teeth, fre- 
quently stimulates an involuntary effort at respiration. To 
secure the tongue if the teeth are clenched, force the jaws 




Fig, 328 



apart with a stick, a piece of wood, or the handle of a pocket 
knife. 

429. Rule (4) — The dashing of cold water into the face 
will sometimes produce a gasp and start breathing, which 
should then be continued as directed above. If this is not 
essential the spine may be rubbed vigorously with a piece of 
ice. Alternate applications of heat and cold over the region 
of the heart will accomplish the same object in some in- 
stances. It is both useless and unwise to attempt to ad- 
minister stimulants to the victim in the usual manner of 
pouring it down his throat. 

While the above directions are being carried out, a physi- 
cian should be summoned, who, upon his arrival, can best put 
into practice Rules (5), (6) and (7), in addition to the fore- 
going, should it be necessary. 



RESUSCITATION 389 

FOR THE PHYSICIAN SUMMONED 

430. Rule (5) — Forcible stretching of the sphincter muscle 
controlling the lower bowel excites powerful refle:: irritation 
and stimulates a gasp (inspiration) frequently when other 
measures have failed. For this purpose the subject should be 
turned on the side, the middle and index fingers inserted 
into the rectum, and the muscle suddenly and forcibly drawn 
backwards toward the spine. Or, if it is desirable to continue 
efforts at artificial respiration at the same time, the knees 
should be drawn up and the thumb inserted for the same 
purpose, the subject retaining the position on the back. 

431. Rule (6) — Rhythmical traction of the tongue is some- 
times effectual in establishing respiration when other meas- 
ures have failed. The tongue is seized and drawn out quickly 
and forcibly to the limit, then it is permitted to recede. This 
is to be repeated 16 times per minute. 

432. Rule (7) — Oxygen gas, which may be readily obtained 
at a drug store in cities or large towns, is a powerful stimu- 
lant to the heart if it can be made to enter the lungs. A 
cone may be improvised from a piece of stiff paper and at- 
tached to the tube leading from the tank, and placed over the 
mouth and nose while the gas is turned on during the efforts 
at artificial respiration. 



CHAPTEK XX 

LOGARITHMS 

433. Definition of Logarithm. — If (a) be any number, and^ 
(x) and (n) two other numbers, such that ax = n, then (x) 
is called the logarithm of (n) to the base (a) and is written 
logaU. The logarithm of a number to a given base is the 
index of the power to which the base must be raised that it 
may be equal to the given number. Example: Since 102 = 
100, therefore 2 =logiolOO. 

434. Laws of Indices. — In algebra the following laws, 
known as the laws of indices, are found to be true; (m) and 
(n) are to be any real quantities: 

(A) am X an = am + 11 

(B) am -^ an =r am — n 

(C) (am)n = amXn 

There are three fundamental laws of logarithms correspond- 
ing to the above. 

(a) loga (m X n) = logam + logaH 

(b) logu (m -^ n) = logam — logan 

(c) logamn = n logam. 
These three laws expressed in words are: 

(a) The logarithm of the product of two quantities is 
equal to the sum of the logarithms of the quantities to the 
same base. 

(b) The logarithm of the quotient of two quantities is 
equal to the difference in their logarithms to the same base. 

(c) The logarithm of a quantity raised to any power is 
equal to the logarithm of the quantity multiplied by the index 
of the power. 

435. Common System of Logarithms. — In what is known 
as the common system of logarithms the base is always 10, 
so that if no base be expressed, the base 10 is always under- 
stood. 

390 



LOGARITHMS 391 

436. Definition of Characteristic and IVIantissa. — If the 

logarithm of any number be partly integral and partly frac- 
tional, the integral portion is called its characteristic and the 
decimal portion is called its mantissa. Thus, the logarithm 
of 666 is 2.82347, in which 2 is the characteristic and 
.82347 is the mantissa. 

The characteristic of the logarithm of any whole number 
will be one less than the number of digits in its integral part. 
Thus, 2457.4 has four digits in' its integral part and the char- 
acteristic of its logarithm is 3. 

The characteristic of the logarithm of any decimal frac- 
tion will be negative and numerically greater by unity than 
the number of ciphers following the decimal point. Thus, 
.897 has no ciphers following the decimal point and the char- 
acteristic will be numerically equal to (0 + 1) or 1, and -it 
will be negative. The fact that the characteristic is negative 
can be indicated by drawing a horizontal line over it, thus 
(1). The characteristic of .00434 is 1 since there are two 
ciphers following the decimal point. 

The mantissa of the logarithm of all numbers consisting 
of the same digits are the same. 

437. How to Obtain the Logarithm of a Number from 
the Table. — For example, if it is desired to obtain the 
logarithm of the number 642, we proceed as follows: Run 
the eye down the extreme left-hand column until it arrives 
at ,the number 64. Then pass along the horizontal line of 
figures until you are in the column vertically beneath the 
number 2 at the top of the page, and you see the number 
80754. This number just obtained is the mantissa of the 
log of 642 and the characteristic is 2. 

Then log 642 = 2.807 54 

and log 6420 = 3.807 54 

log 6.42= .807 54 

log .00642 = 3.807 54 

438. To Find a Number Whose Logarithm Is Given. — If the 

logarithm be one tabulated in the table the number is easily 
found, the procedure being just the reverse of that given in 
the previous paragraph. Example: Find the number whose 
logarithm is 68931. Referring to the table, we find the 




392 PRACTICAL APPLIED ELECTRICITY 

mantissa 68931 corresponds to the digits 489, and the numbef 
will be 4890, since the characteristic is 3. 

Often the logarithm is not tabulated and the number is 
then found as follows: Example — Find the number whose 
logarithm is 3.44741. Referring to the table, we find that the 
mantissa 44741 is not tabulated, but the nearest mantissae 
are 44716 and 44871, between which, the mantissa 44741 lies. 
The difference between 44871 and 44716 is 155, and the differ- 
ence between 44741 and 44716 is 75. 

log 2800 = 3.447 16 

log 2810 = 3.448 71 
then 3.447 41 = log (2800 + X) 

75 
X = — of 10 = 4.99 

155 

The require^ number then is (2800 + 4.99) = 2804.99. 



EXAMPLES 

(A) What is the value of the product of 24 and 19? 
log 24 = 1.386 21 
log 19 = 2.278 75 



log (product) = 3.658 96 
then (24 X 19) = 456 

(B) "What is the value of (27)2? 

log (27)2 = 2 log 27 
= 2.862 72 
then (27)2 = 729 

(C) What is the value of the product of .079 and .03? 

log .079 = 27897 63 
log .03 =2147712 



log (product) = 3.374 75 (See note a, page 
then (.079 X .03) = .002 37 393) 



^ 



. LOGARITHMS 393 

(D) What is the value of the product of .65 and 48? 

log .65 =1.812 91 
log 48 -= 1.681 24 



log (product) = 1.494 15 
then (.65 X 48) ==31.25 

(E) What is the value of (.075)2? 

log .075 = 2:875 06 

2 log .075 = 3.750 12 

then (.075)2= .005 625 

(F) What is the value of (79.0)1.6? 

log 79.0 = 1.897 63 
log (79.0)1.6 = 1.6 X 1.8*7 63 
= 2.936 208 
then (79.0)1.6 = 863.+ 



(G) What is the value of V 656100 ? 

log 656100 = 5.816 90 

Vz log 656100 = 2.908 45 



or log V 656100 = 2.908 45 



then V 656100 = 810 

Note (a) — The characteristic is treated as a negative quan- 
tity and the mantissa as a positive quantity. 



394 



LOGARITHMS OF NUMBERS 



Xo. 





1 


2 


3 


4 


5 


« 


7 


8 


9 





000 00 301 03 477 12 602 06 698 97 


778 15 


845 10 


903 09 


954 24 


1 


000 00 


041 39 


079 18 


113 94 


146 13 


176 09 


204 12 


230 45 


255 27 


278 75 


2 


30103 


322 22 


342 42 


36173 


380 21 


397 94 


414 97 


43136 


447 16 


462 40 


3 


477 12 


491 36 


505 15 


518 51 


53148 


544 07 


556 30 


568 20 


579 78 


59106 


4 


602 06 


612 78 


623 25 


633 47 


643 45 


65321 


662 76 


672 10 


681 24 


690 20 


5 


698 97 


707 57 


716 00 


724 28 


732 39 


740 36 


748 19 


755 87 


763 43 


770 85 


6 


778 15 


785 33 


792 39 


799 34 


806 18 


812 91 


819 54 


826 07 


832 51 


818 85 


7 


845 10 


85126 


857 33 


863 32 


869 23 


875 06 


880 81 


886 49 


892 09 


897 63 


8 


903 09 


908 49 


913 81 


919 08 


924 28 


929 42 


934 50 


939 52 


944 48 


949 39 


9 


954 24 


959 04 


963 79 


968 48 


973 13 


977 72 


982 27 


986 77 


99123 


995 64 


10 


000 00 


004 32 


008 60 


012 84 


017 03 


02119 


025 31 


029 38 


033 42 


037 43 


11 


04139 


045 32 


049 22 


053 08 


056 90 


060 70 


064 46 


068 19 


07188 


075 55 


12 


07918 


082 79 


086 36 


089 91 


093 42 


096 91 


100 37 


103 80 


107 21 


110 59 


13 


113 94 


117 27 


120 57 


123 85 


127 10 


130 33 


133 54 


136 72 


139 88 


143 01 


14 


146 13 


149 22 


152 29 


155 34 


158 36 


16137 


164 35 


167 32 


170 26 


173 19 


15 


176 09 


178 98 


18184 


184 69 


187 52 


190 33 


193 12 


195 90 


198 66 


20140 


16 


204 12 


206 83 


209 52 


212 19 


214 84 


217 48 


^220 11 


222 72 


225 31 


227 89 


17 


230 45 


233 00 


235 53 


238 05 


240 55 


243 04 


245 51 


247 97 


250 42 


252 85 


18 


255 27 


257 68 


260 07 


262 45 


264 82 


267 17 


269 51 


27184 


274 16 


276 46 


19 


278 75 


28103 


283 30 


285 56 


287 80 


290 03 


292 26 


294 48 


296 67 


298 85 


20 


30103 


303 20 


305 35 


307 50 


309 63 


31175 


313 87 


315 97 


318 06 1 320 15 


21 


322 22 


324 28 


326 34 


328 38 


330 41 


332 44 


334 45 


336 46 


338 46 


340 44 


22 


342 42 


344 39 


346 35 


348 30 


350 25 


352 18 


354 11 


356 03 


357 93 


359 84 


23 


36173 


363 61 


365 49 


367 36 


369 22 


37107 


372 91 


374 75 i 376 58 


378 40 


24 


380 21 


382 02 


383 82 


385 61 


387 39 


389 17 


390 94 


392 70 


394 45 


396 20 


25 


397 94 


399 67 


40140 


403 12 


404 83 


406 54 


408 24 


409 93 


41162 


413 30 


28 


414 97 


416 64 


418 30 


419 96 


42160 


423 25 


424 88 


426 51 


428 13 


429 75 


27 


43136 


432 97 


434 57 


436 16 


437 75 


439 33 


440 91 


442 48 


444 04 


445 60 


23 


447 16 


448 71 


450 35 


45179 


453 32 


454 84 


456 37 


457 88 


459 39 


460 90 


29 


462 40 


463 89 


465 38 


466 87 


468 35 


469 82 


47129 


472 76 


474 22 


475 67 


30 


477 12 


478 57 


480 01 


48144 


482 87 


484.30 


485 72 ' 487 14 


488 55 


489 96 


31 


49136 


492 76 


494 15 


495 54 


496 93 


498 31 


499 69 


50106 


502 43 


503 79 


32 


505 15 


506 51 


507 86 


509 20 


510 55 


51188 


513 22 


514 55 


515 87 


517 20 


33 


518 51 


519 83 


52114 


522 44 


523 75 


525 04 


526 34 


527 63 


528 92 


530 20 


34 


53148 


532 75 


534 03 


535 29 


536 56 


537 82 


539 08 


540 33 


54158 


542 83 


35 


544 07 


545 31 


546 54 


547 77 


549 00 


550 23 


55145 


552 67 


553 88 


555 09 


36 


556 30 


557 51 


558 71 


559 91 


561 10 


562 29 


563 48 


564 67 


565 85 


567 03 


37 


568 20 


569 37 


570 54 


57171 


572 87 


574 03 


575 19 


576 34 


577 49 


578 64 


38 


579 78 


580 92 


582 06 


583 20 


584 33 


585 46 


586 59 


587 71 


588 83 


589 95 


39 


59106 


592 18 


593 29 


594 39 


595 50 


596 60 


597 70 


598 79 


599 88 


600 97 


40 


602 06 


603 14 


604 23 


605 31 


606 38 


607 46 


608 53 


609 59 


610 66 


61172 


41 


612 78 


613 84 


614 90 


615 95 


617 00 


618 05 


619 09 


620 14 


621 18 


622 21 


42 , 


623 25 


624 28 


625 31 


626 34 


627 37 


628 39 


629 41 


630 43 


63144 


632 46 


43 1 


633 47 


634 48 


635 48 


636 49 


637 49 


638 49 


639 49 


640 48 


64147 


642 46 


44 , 


643 45 


644 44 


645 42 


646 40 


647 38 


648 36 


649 33 


650 31 


651 28 


652 25 


45 


653 21 


654 18 


655 14 


656 10 


657 06 


658 01 


658 96 


659 92 


660 87 


66181 


46 , 


662 76 


663 70 


664 64 


665 58 


666 52 


667 45 


668 39 


669 32 


670 25 


671 17 


47 ! 


672 10 


673 02 


673 94 


674 86 


675 78 


676 69 


677 61 


678 52 


679 43 


680 34 


48 


68124 


682 15 


683 05 


683 95 


684 85 


685 74 


686 64 


687 53 


688 42 


689 31 


49 


090 20 


69108 


69197 


692 85 


693 73 


694 61 


695 48 


696 36 


697 23 


698 10 


50 


698 97 


699 84 


700 70 


70157 


702 43 


703 29 

1 


704 15 


705 01 


705 86 


706 72 



LOGARITHMS OF NUMBERS 



39c 



No. 





1 


2 


3 


4 


5 


6 


7 


8 


9 


51 


707 57 


708 42 


709 27 


710 12 


710 96 


71181 


712 65 


712 49 


714 33 


715 17 


52 


716 00 


716 84 


717 67 


718 50 


719 33 


720 16 


720 99 


72181 


722 63 


723 46 


53 


724 28 


725 09 


725 91 


726 73 


727 54 


728 35 


729 16 


729 97 


730 78 


73159 


54 


732 39 


733 20 


734 00 


734 80 


735 60 


736 40 


737 19 


737 99 


738 78 


739 57 


55 


740 36 


741 15 


74194 


742 73 


743 51 


744 29 


745 07 


745 86 


746 63 


747 41 


56 


748 19 


748 96 


749 74 


750 51 


75128 


752 05 


752 82 


753 58 


754 35 


755 11 


57 


755 87 


756 64 


757 40 


758 15 


758 91 


759 67 


760 42 


76118 


76193 


762 68 


58 


763 43 


764 18 


764 92 


765 67 


766 41 


767 16 


767 90 


768 64 


769 38 


770 12 


59 


770 85 


77159 


772 32 


773 05 


773 79 


774 52 


775 25 


775 97 


776 70 


777 43 


60 


778 15 


778 87 


779 60 


780 32 


78104 


78176 


782 47 


783 19 


783 90 


784 62 


61 


785 33 


786 04 


786 75 


787 46 


788 17 


788 88 


789 58 


790 29 


790 99 


79169 


62 


792 39 


793 09 


793 79 


794 49 


795 18 


795 88 


796 57 


797 27 


797 96 


798 65 


63 


799 34 


800 03 


800 72 


80140 


802 09 


802 77 


803 '46 


804 14 


804 82 


805 50 


64 


806 18 


806 86 


807 54 


808 21 


808 89 


809 56 


810 23 


810 90 


81158 


812 24 


65 


812 91 


813 58 


•814 25 


814 91 


815 58 


816 24 


816 90 


817 57 


818 23 


818 89 


66 


819 54 


820 20 


820 86 


82151 


822 17 


822 82 


823 47 


824 13 


824 78 


825 43 


67 


826 07 


826 72 


827 37 


828 02 . 


828 66 


829 30 


829 95 


830 59 


83123 


83187 


68 


832 51 


833 15 


833 78 


834 42 


835 06 


835 69 


836 32 


836 96 


837 59 


838 22 


69 


838 85 


839 48 


840 11 


840 73 


84136 


84198 


842 61 


843 23 


'843 86 


844 48 


70 


845 10 


845 72 


846 34 


846 96 


847 57 


848 19 


848 80 


849 42 


850 03 


850 65 


71 


85126 


85187 


852 48 


853 09 


853 70 


854 31 


854 91 


855 52 


856 12 


856 73 


72 


857 33 


857 94 


858 54 


859 14 


859 74 


860 34 


860 94 


86153 


862 13 


862 73 


73 


863 32 


863 92 


864 51 


865 10 


865 70 


866 29 


866 88 


867 47 


868 06 


868 64 


74 


869 23 


86^82 


870 40 


870 99 


87157 


872 16 


872 74 


873 32 


873 90 


874 48 


75 


875 06 


875 64, 


876 22 


876 79 


877 37 


877 95 


878 52 


879 10 


879 67 


880 24 


76 


880 81 


88tS8t 


88195 


882 52 


883 19 


883 66 


884 23 


884 80 


885 36 


885 93 


77 


886 49 


887 OS^ 


887 62 


888 18 


888 74 


889 30 


889 86 


890 42 


890 98 


89154 


78 


892 09 


smm 


893 21 


893 76 


894 32 


894 87 


895 42 


895 97 


896 53 


897 08 


79 


897 63 


898^1^ 


898 73 


899 27 


899 82 


900 37 


900 91 


90146 


902 00 


902 55 


80 


903 09 


903 63 


904 17 


904 72 


905 26 


905 80 


906 34 


906 87 


907 41 


907 95 


81 


908.49 


909 02 


909 56 


910 09 


910 62 


91116 


91169 


912 22 


912 75 


913 28 


82 


913 81 


914 34 


•914 87 


915 40 


915 93 


916 45 


916 98 


917 51 


918 03 


918 55 


83 


919 08 


919 60 


920 12 


920 65 


921 17 


92169 


922 21 


922 73 


923 24 


923 76 


84 


924 28 


924 80 


925 31 


925 83 


926 34 


926 86 


927 37 


927 88 


928 40 


928 91 


85 


929 42 


929 93 


930 44 


930 95 


93146 


93197 


932 47 


932 98 


933 49 


933 99 


86 


934 50 


935 00 


935 51 


936 01 


936 51 


937 02 


937 52 


938 02 


938 52 


939 02 


87 


939 52 


940 02 


940 52 


94101 


94151 


942 01 


942 50 


943 00 


943 49 


943 99 


88 


944 48 


944 98 


945 47 


945 96 


946 45 


946 94 


947 43 


947 92 


948 41 


948 90 


89 


949 39 


949 88 


950 30 


950 85 


95134 


95182 


952 31 


952 79 


9.^3 28 


953 76 


90 


954 24 


954 72 


955 21 


955 69 


956 17 


956 65 


957 13 


957 61 


958 09 


958 56 


91 


959 04 


959 52 


959 99 


960 47 


960 95 


96142 


96190 


962 37 


962 84 


963 32 


92 


963 79 


964 26 


964 73 


965 20 


965 67 


966 14 


966 61 


967 08 


967 55 


968 02 


93 


963 48 


968 95 


969 42 


969 88 


970 35 


970 81 


97128 


97174 


972 20 


972 67 


94 


973 13 


973 59 


974 05 


974 51 


974 97 


975 43 


975 89 


976 35 


976 81 


977 27 


95 


977 72 


978 14 


978 64 


979 09 


979 55 


980 00 


980 46 


980 91 


981 37 ' 981 82 


96 


982 27 


982 72 


983 18 


983 63 


984 08 


984 53 


984 98 


985 43 


985 88 


986 32 


97 


986 77 


987 22 


987 67 


988 11 


988 56 


989 00 


989 45 


989 89 


990 34 


990 78 


98 


99123 


99167 


992 11 


992 55 


993 00 


993 44 


993 88 


994 32 


994 76 


995 20 


99 


995 64 


996 07 


996 51 


996 95 


997 39 


997 82 


998 26 


998 70 


999 13 


999 57 


100 


000 00 


000 43 


000 87 


00130 


00173 


002 17 


002 60 


003 03 


003 46 


003 89 



?96 



MENSURATION EQUATIONS 




TRIGONOMETRICAL FUNCTIONS 
a. 

MENSURATION 



Sine 6=1; Cosinee='p Tangent e="c' 



T 

h 

J.. 



Rectangle 



Area=bxl 



-L 



Square 



Area=bxb = b 



soTri angle 



b 

Base = b 

Altitude = a 

Hypot enuse = h 

b=Y?Tb33 

b=Vb"-a^ 
Area= /axaxb 

? /Any\ 

j, XTri angle \ 

Base =b 

Altitude'' a 

Area= ^kxaxb 




Diameter = d 

Radiu5 = r=d^a 

Circumference ^nxd 

rT= 3.1416- 

Area = Trxr^ 

= Vat xn xd' 



^ 




1 

J- - - 


y 


A 


k 1 > 


<b 




Length =1 

Breath =b 

Depth = d 

Volume =lxbxd 

Area surface = 

2(dxb) + a(bxl)+a(dxl) 



Area Shaded 
Portion=J[]^(d|-d^) 





l=b = d 
Volume =lx b xd. 



d ^P Cylinder j 



Area=/2lxr 



, xrrxr 




Area Shaded 
Portion /2[lr-c(r-h)]= 
3fo xnxr^" c(r-h) 
c=chord = 2V2hr-h2 



Surface = 
TTxd^ = 4TTxr^ 

Volume = 
y6TTxd^=y3nxr^ 



TRIGONOMETKICAL FUNCTIONS 



397 



NATURAL SINES, COSINES, AND TANGENTS 



An- 








An- 








gle 


Sines 


Cosines 


Tangents 


gle 


Sines 


Cosines 


Tangents 





.000 000 


1.000 000 


.000 000 


46 


.719 340 


.694 658 


1.035 530 3 


1 


.017 452 


.999 848 


.017 455 


47 


.731 354 


.681 998 


1.072 368 7 


2 


.034 899 


.999 391 


.034 921 


48 


.743 145 


.669 131 


1.110 612 5 


3 


.052 336 


.998 630 


.02 408 


49 


.754 710 


.656 059 


1.150 368 4 


4 


.069 756 


.997 564 


.069 927 


50 


.766 044 


.642 788 


1.191753 6 


5 


.087 156 


.996 165 


.087 489 


51 


.777 146 


.629 320 


1.234 897 2 


6 


.104 528 


.994 522 


.105 104 


52 


.788 Oil 


.615 661 


1.279 9416 


7 


.121869 


.992 546 


.122 785 


53 


.798 636 


.601815 


1.327 044 8 


8 


.139 173 


.990 268 


.140 541 


54 


.809 017 


.587 785 


1.376 3810 


9 


.156 434 


.987 688 


.158 384 


55 


.819 152 


.573 576 


1.428 148 


10 


.173 648 


\ 984 808 


.176 327 


56 


.829 038 


.559 193 


1.482 5610 


11 


. 190 809 


.981 627 


.194 380 


57 


.838 671 


.544 639 


1.539 865 


12 


.207 912 


.978 148 


.212 557 


58 


.848 048 


.529 919 


1.600 334 5 


13 


.224 951 


.974 370 


.230 868 


59 


.857 167 


.515 038 


1.664 279 5 


14 


.241922 


.970 296 


.249 328 


60 


.866 025 


.500 000 


1.732 050 8 


15 


.258 819 


.965 926 


.267 949 


61 


.874 620 


.484 810 


1.804 047 8 


16 


.275 637 


.961 262 


.286 745 


62 


.882 948 


.469 472 


1.880 726 5 


17 


.292 372 


.956 305 


.305 731 


63 


.891 007 


.453 990 


1.962 610 5 


18 


.309 017 


.951 057 


.324 920 


64 


.898 794 


.438 371 


2.050 303 8 


19 


.325 568 


.945 519 


.344 328 


65 


.906 308 


.422 618 


2.144 506 9 


20 


.342 020 


.939 693 


.363 970 


66 


.913 545 


.406 737 


2.246 036 8 


21 


.358 368 


.933 580 


.383 864 


67 


.920 505 


.390 731 


2.355 852 4 


22 


.374 607 


.927 184 


.404 026 


68 


.927 184 


.374 607 


2.475 086 9 


23 


.390 731 


.920 505 


.424 475 


69 


.933 580 


.358 369 


2.605 089 1 


24 


.406 737 


.913 545 


.445 229 


70 


.939 693 


.342 020 


2.747 477 4 


25 


.422 618 


.906 308 


.466 308 


71 


.945 519 


.325 568 


2.904 210 9 


26 


.438 371 


.898 794 


.487 733 


72 


.951 057 


.309 017 


3.077 683 51 
3.270 852 61 


27 


.453 990 


.891007 


.509 525 


73 


.956 305 


.292 372 


28 


.469 472 


.882 948 


.531 709 


74 


.961 262 


.275 637 


3.487 414 4 


29 


.484 810 


.874 620 


.554 309 


75 


.965 926 


.258 819 


3.732 050 8 < 
4.010 780 9 


30 


.500 000 


.866 025 


.577 350 


76 


.970 296 


.241 922 


31 


.515 038 


.857 167 


.600 861 


77 


.974 370 


.224 951 


4.331475 9, 


32 


.529 919 


.848 048 


.624 869 


78 


.978 148 


.207 912 


4.704 730 1 


33 


.544 639 


.838 671 


.649 408 


79 


.981 627 


.190 809 


5.144 554 


34 


.559 193 


.829 038 


.674 509 


80 


.984 808 


.173 648 


5.6712818 


35 


.573 576 


.819 152 


.700 208 


81 


.987 688 


.156 434 


6.313 7515 


36 


.587 785 


.809 017 


.726 543 


82 


.990 268 


.139 173 


7.115 369 7 


37 


.601 815 


.798 636 


.753 554 


83 


.992 546 


.121869 


8.144 346 4 


38 


.615 661 


.788 011 


.781 286 


84 


.994 522 


.104 528 


9.514 364 5 


39 


.629 320 


.777 146 


.809 784 


85 


.996 195 


.087 156 


11.430 052 


40 


.642 788 


.766 044 


.839 100 


86 


.997 564 


.069 756 


14.300 666 


41 


.656 059 


.754 710 


.869 287 


87 


.998 630 


.052 336 


19.081 137 


42 


.669 131 


.743 145 


.900 404 


88 


.999 391 


.034 899 


28.636 253 


43 


.681 998 


.731 354 


.932 515 


89 


.999 848 


.017 452 


57.289 962 


44 


.694 658 


.719 340 


.965 689 


90 


1.000 000 


.000 000 


Infinite 


45 


.707 107 


.707 170 


1.000 000 











308 



SYMBOLS FOB ELECTRICAL APPARATUS 



n 



Wfre 



Wires not 
Connected 



Wires 
Connected 



Ground 
Connection 



Fuse 



— vwwv 

Non-inductive 
Resistance 



TRnRR 

Inductive 
Resistance 

W/^ j 

Variable 

Resistance or 

Rheostat 



Incandescent 
Lamps in Series 

X — X— X— X- 

Arc Lamps 
in Series 



hK- 



PrinnaryCell 

— H 



Double-Pole Double 
Throw Switch (Open) 



Storage Cell 



^3: 



Pole Changer 



^ 



Ammeter 



j&r 



oV 
o * . 



Direct -Current 
Generator 



Voltmeter 



jEf 



.®. 



Galvanometer 



Direct-Current 
Motor 



Load 



rO 



w 



Wattmeter 



Alternating- 
Current Generator 



.<j::^^ 



Simple Switch 



— '^wmi^ 

Transformer 



Single-Pole Single 
Throw Switch (Open) 



Condenser 



Smgle-Pole Double 

'Throw Switch(Open) Q^^a Connection 





Double-Pole Single- 

Throw Switch (Open) Star Connection 



TABLE A 



399 



RELATION OF METRIC AND ENGLISH MEASURES 
Equivalents of Linear Measures 





Meters 


English Measures 




Inches 


Feet 


Yards 


Miles 


Millimeter . . 


.001 
.01 
.1 
1. 
10. 
100. 
1000. 


.039 371 

3.937 07 J 

.328 089 

39.370 790 


.003 281 
.032 809 
.328 089 
3.280 899 
32.808 99 
328.089 9 
3 280.899 


.001 094 
.010 936 
. 109 363 
1.093 633 
10.936 33 
109.363 3 
1093.633 




Centimeter 

Decimeter 

Meter 


.000 621 


Decameter 

Hectometer 

Kilometer 


.006 214 
.062 1S8 
.621 382 



English Measures 


Meters 


Reciprocals 




.02539954 

.3047945 

.9143835 


39.37079 




3 280899 


3 feet=l yard 


1.093633 





Equivalents of Surface Measures 










Square 
Meters 


English Measures 




Square 
inches 


Square 
feet 


Square 
yards 


Milliare. . . 


.1 

10 ; 

100. 

1000. 

10 000. 

1000 000. 


155.01 
1550.06 
15500.59 
155005.9 


1.076 
10.764 
107.64 
1076.4 
10764.3 
107643. 


119 


Centiare 


1.196 


Diciare 


11 960 


Are 

Decare (not used) 

Hectare 

Square Kilometer 


119.6033 
1096.033 
11960.33 



English Measures 



1 square inch. . . . ... 

144 sq. in.=l sq. ft 
9 sq. ft.=l sq. yd-. 



Metric Measures 



6.451367 sq. cmt. 
.09289968 sq. m. 
.8360972 sq. m. 



Reciprocals 



. 1550059 

10.7642996 

1.196033 



Equivalent; of Weights 





Grams 


English Weights 




Oz. 
avoir 


Lbs. Tons 
avoir 2000 lbs. 


Tons 
2240 lbs. 


Milligram 

Centigram 

Decigram 

Gram.. 


.001 
.01 
.1 
1. 
10. 
100. 
1000. 


.0353 

.3527 

3.5274 

35.2739 


.0022 
.02205 
.22046 
2.2046 


.001102 




Decagram 

Hectogram 

Kilogram 


.000984 





English Weights "Avoirdupois" 


Grants 


Reciprocals 


1 grain 

24 34375 grains — 1 dram 


.06479895 
1.771836 
28.349375 
453.592652 


15.43234875 
.564383 


16 drams=l oz.==437.5 grains. . . . . i 

16 oz.l pound==7000 grains 


.0352739 
.00220462 



400 



TABLE B 



TABLE OF COMPARISON OF CENTIGRADE AND FAHRENHEIT 

THERMOMETER SCALES 



Cent. 


Fahr. 


Cent. 


Fahr. 


Cent. 


Fahr. 


Cent. 


Fahr. 





32.0 


26 


78.8 


51 


123.8 


76 


168.8 


1 


33.8 


27 


80.6 


52 


125.6 


77 


170.6 


2 


35.6 


28 


82.4 


53 


127.4 


78 


172.4 


3 


37.4 


29 


84.2 


54 


12^.2 


79 


179.2 


4 


39.2 


30 


86.0 


55 


131.0 


80 


176.0 


5 


41.0 


31 


87.8 


56 


132.8 


81 


177.8 


6 


42.8 


32 


89.6 


57 


134.6 


82 


179.6 


7 


44.6 


33 


91.4 


58 


136.4 


83 


181.2 


8 


46.4 


34 


93.2 


59 


138.2 


84 


183.4 


9 


48.2 


35 


95.0 


60 


140.0 


85 


185.0 


10 


50.0 


36 


96.8 


61 


141.8 


86 


186.8 


11 


51.8 


37 


98.6 


62 


143.6 


87 


188.6 


12 


53.6 


38 


100.4 


63 


145.4 


88 


190.4 


13 


55.4 


39 


102.2 


64 


147.2 


89 


192.2 


14 


57.2 


40 


104.0 


65 


149.0 


90 


194.0 


15 


59.0 


41 


105.8 


66 


150.8 


91 


195.8 


16 


60.8 


42 


107.6 


67 


152.6 


92 


197.6 


17 


62.6 


43 


109.4 


68 


154.4 


93 


199.4 


18 


64.4 


44 


111.2 


69 


156.2 


94 


201.2 


19 


66.2 


45 


113.0 


70 


158.0 


95 


203.0 


20 


68.0 


46 


114.8 


71 


159.8 


96 


204.8 


21 


69.8 


47 


116.6 


72 


161.6 


97 


206.6 


22 


71.6 


48 


118.4 


73 


163.4 


98 


208.4 


23 


73.4 


49 


120.2 


74 


165.2 


99 


210.2 


24 


75.2 


50 


122.0 


75 


167.0 


100 


212.0 


25 


77.0 















One deg. Fahr.=.5556 deg. centigrade. 
One deg. centigrade=1.8 deg. Fahr. 

To convert Fahr. to centigrade, subtract 32, multiply by 5 and divide by 9. 
To convert centigrade to Fahr., multiply by 9, divide by 5 and add 32. 
If temperature is below freezing, the above formula should read "subtract from 32" in place of 
"subtract 32" and "add 32." 

TABLE C 
EQUIVALENT CROSS-SECTIONS OF DIFFERENT SIZE WIRES 
(Brown and Sharpe Gauge) 







Number of 


wires of various sizes 




Equiv. 














section 


















2 


4 


8 


16 


32 


64 


128 


0000 





3 


6 


9 


12 


15 


18 


000 


i. 


4 


7 


10 


13 


16 


One each 


CO 


2 


5 


8 


11 


.14 


17 


1 and 3 





3 


6 


9 


12 


15 


18 


2 and 4 


1 


4 


7 


10 


13 


16 




3 and 5 


2 


5 


8 


11 


14 


17 




4 and 6 


3 


6 


9 


12 


15 


18 




5 and 7 


4 


7 


10 


13 


16 






6 and 8 


5 


8 


11 


14 


17 






7 and 9 


6 


9 


12 


15 


18 






8 and 10 


7 


10 


13 


• 16 








9 and 11 


8 


11 


14 


17 








10 and 12 


9 


12 


15 


18 








11 and 13 


10 


13 


16 










12 and 14 


11 


14 


17 










13 and 15 


12 


15 


18 










14 and 16 


13 


16 












15 and 17 


14 


17 












16 and 18 


15 


18 















TABLE D 

COMPARATIVE TABLE OF WIRE GAUGES 



401 



Gauge 
No. 


American Wire Gauge 
(Brown & Sharre) 


Birmingham Wire Gauge 
(Stubs) 


Standard Wire Gauge 


Diameter 


Area 


Diameter 


Area 


Diam'ter 


\rea 




Inches 


Circular 
Mills 


Inches 


Circular 
Mills 


Inches 


Circular 
Mills 


7-0 






0;45i""* 
0.425 


20Q'm.' 
180 600. 


0.500 
0.404 
0.432 
0.400 
0.372 


2 50000 


6-0 






2 15300. 


5-0 






1 86600 


4-0 
3-0 


0.460 
0.409 6 


211 600. 
167 800. 


1 60000. 
1 38400. 


2-0 

1-0 

1 

2 

3 


0.364 8 
0.324 9 
0.289 3 
0.257 6 
0.229 4 


133 100. 
105 500. 

83 690. 

66 370. 

52 630. 


0.380 
0.340 
0.300 
0.284 
0.259 


144 400. 
115 600 

90 000. 

80 660. 

67 080. 


0.348 
0.324 
0.300 
0.276 
0.252 


1 21100. 
1 05000. 

90000. 

76180. 

63500. 


4 
5 
6 
7 
8 


0.204 3 
0.1819 
0.162 
0.144 3 
0.128 5 


41 740. 
33 100 
26 250. 
20 820. 
16 510. 


0.238 
0.220 
0.203 
0.180 
0.165 


56 640. 
48 400. 
41210. 
32 400. 
27 230. 


0.232 
0.212 
0.192 
0.176 
0.160 


53820. 
44940. 
36860. 
30980. 
256o0. 


9 
10 
11 
12 
13 


0.114 4 
0.1019 
090 74 
0.080 81 
0.071 96 


13 090. 
10 380. 

8 234. 

6 530. 

5 178. 


0.148 
0.134 
0.120 
0.109 
0.095 


21900. 
17 960. 
14 400. 
11 880. 
9 025. 


0.144 
0.128 
0.116 
0.104 
0.092 


20740. 
16380. 
13460. 
10820. 
8464. 


14 
15 
16 
17 
18 


0.064 08 
0.057 07 
0.050 82 
0.045 26 
0.040 30 


4 107. 
3 257. 
2 583. 
2 048. 
1624. 


0.083 
0.072 
0.065 
0.058 
0.049 


6 889. 
5 184. 
4 225. 
3 364. 
2 401. 


0.080 
0.072 
0.064 
0.056 
0.048 


6400. 
5184. 
4096. 
3136. 
2304. 


19 
20 
21 
22 
23 


0.035 89 
0.03196 
0.028 46 
0.025 35 
0.022 57 


1288. 

1022. 
810.1 
642.4 
509.5 


0.042 
0.035 
0.032 
0.028 
0.025 


1764. 
1225. 
1024. 

784. 
625. 


0.040 
0.036 
0.032 
0.028 
0.024 


1600. 

1296. 

1024. 
784.0 
576.0 


24 
25 
26 
27 

28 


0.020 10 
0.017 90 
0.015 94 
0.014 20 
0.012 64 


404.0 
320.4 
254.1 
201,5 
159.8 


0.022 
0.020 
0.018 
0.016 
0.014:0 


484. 
400. 
324. 
256. 
196. 


0.022 
0.020 
0.018 
0.016 4 
0.014 8 


484.0 
400.0 
324.0 
269.0 
219.0 


29 
30 
31 
32 
33 


0.01126 
0.01O03 
0.008 928 
0.007 950 
0.007 080 


126.7 
100.5 
79.70 
63.21 
50.13 


0.013 
0.012 
0.010 
0.009 
0.008 


169. 
144. 
100. 

81. 

64. 


0.013 6 
0.012 4 
0,0116 
0.0108 
0.010 


185.0 
153.8 
134.6 
116.6 
100.0 


34 
35 
36 
37 
38 


0.006 305 
0.005 615 
0.005 000 
0.004 453 
0.003 965 


39.75 
31.52 
25.00 
19.83 
15.72 


0.007 
0.005 
0.004 4 


49. 
25. 
16. 


0.009 2 
0.008 4 
0.007 6 
0.006 8 
0.006 


84.64 
70.56 
67.76 
46.24 
36.00 


39 
40 
41 


0.003 531 
0.003145 


12.47 

9.888 







0.005 2 
0.004 8 
0.004 4 


27.04 
23.04 
19.36 



402 



TABLE E 



COPPER WIRE TABLE OF AMERICAN 
Giving Weights and Lengths of cool, warm and hot wires. 



B.&S. 


Diameter 


Area 




Lbs. Per 




or 








Foot 




A. W. G. 














Circ'jiar 


Sq. in. 




68° F. 




Inches 


miles 


Sq. miles 




20° C. 


0000 


0.460 


211600 


166 190 


0.640 5 


13 090 


000 


0.409 6 


167 800 


131 790 


0.508 


8 232 


00 


0.364 8 


133 100 


104 518 


0.402 8 


5 177 





0.324 9 


105 500 


82 887 


0.319 5 


3 256 


1 


0.289 3 


83 690 


65 732 


0.253 3 


2 048 


2 


0.257 6 


66 370 


52 128 


0.2009 


1288. 


3 


0.229 4 


52 630 


41339 


0.159 3 


810.0 


4 


0.204 3 


41740 


32 784 


0.126 4 


509.4 


5 


0.1819 


33 100 


25 999 


0.1002 


320.4 


6 


0.162 


26 250 


20 618 


0.079 46 


2015 


7 


0.144 3 


20 820 


16 351 


0.630 2 


126.7 


8 


0.128 5 


16 510 


12 967 


0.049 98 


79.69 


9 


0.1144 


13 090 


10 283 


0.039 63 


50.12 


10 


0.1019 


10 380 


8 155 


0.03143 


31.52 


11 


0.090 74 


8 234 


6 467 


0.024 93 


19.82 




0.080 81 


6 530 


5 129 


0.019 77 


12.47 


1312 


0.07196 


5 178 


4 067 


0.015 68 


7.840 


14 


0.064 08 


4 107 


3 225 


0.012 43 


4.931 


15 


0.057 07 


3 257 


2 558 


0.009 85S 


3.101 


16 


0.050 82 


2 583 


2 029 


0.007 818 


1.950 


17 


0.045 26 


2 048 


1609 


0.006 200 


1.226 


18 


0.040 30 


1624 


1276 


0.004 917 


0.771 3 


19 


0.035 89 


1288 


1012 


0.003 899 


0.485 1 


20 


0.031 96 


1022 


802 


0.003 092 


0.305 1 


21 


0.028 46 


810.1 


632.3 


0.002 452 


0.1919 


22 


0.025 35 


642.4 


504.6 


0.001945 


120 7 


23 


0.022 57 


509.5 


400.2 


0.001 542 


0.075 89 


24 


0.020 10 


404.0 


317.3 


0.001223 


047 73 


25 


0.017 90 


324.4 


251.7 


0.000 969 9 


0.030 02 


26 


0.015 94 


254.1 


199.6 


0.000 769 2 


0.018 88 


27 


0.014 2 


201.5 


158.3 


0.000 610 


0.011 87 


28 


0.012 64 


159.8 


125.5 


0.000 483 7 


0.007 466 


29 


0.01126 


126.7 


99.53 


0.000 383 6 


004 696 


30 


0.010 03 


100.5 


78.94 


0.000 304 2 


0.002 953 


31 


0.008 928 


79.70 


62.60 


0.000 241 3 


0.001857 


32 


0.007 950 


63.21 


49.64 


0.000 1913 


' 0.001 168 


33 


0.007 080 


50.13 


39.37 


0.000 151 7 


000 734 6 


34 


0.006 305 


39.75 


31.22 


0.000 120 3 


0.000 462 


35 


0.005 615 


31.52 


24.76 


0.000 095 43 


0.000 2905 


36 


0.005 


25.0 


19.64 


0.000 075 68 • 


0.000 182 7 


37 


0.004 453 


19.83 


15.57 


0.000 060 01 


0.000 114 9 


38 


0.003 965 


15.72 


IJ^.35 


0.000 047 59 


0.000 072 10 


39 


0.003 531 


12.47 


9.79 


0.000 037 74 


0.000 045 45 


40 


0.003 145 


9.888 


7.77 


0.000 029 93 


0.000 028 58 



TABLE E 



403 



INSTITUTE OF ELECTRICAL ENGINEERS 
of Matthiessen's standard of conductivity. 



lbs 


. per 


Feet per 




Feet Per 




Ohm. 


Pound 




ohm 




122° F. 


176° F. 




68° F. 


122° F. 


176° F. 


50° C. 


80° C. 




20° C. 


50° C. 


80° C. 


11720 


10 570 


1.561 


20 440 


18 290 


16 510 


7 369 


6 647 


1.969 


16 210 


14 510 


13 090 


4 634 


4 182 


2.482 


12 850 


11500 


10 380 


2 914 


2 630 


3 130 


10 190 


9 123 


8 232 


1 833 


1654 


3.947 


8 083 


7 235 


6 528 


1153 


1040 


4.977 


6 410 


5 738 


5 177 


725.0 


654.2 


6.276 


5 084 


4 550 


4 106 


455.9 


411.4 


7.914 


4 031 


3 608 


3 256 


286.7 


258.7 


9.980 


3 1<^7 


2 862 


2 582 


180.3 


162.7 


12.58 


2 535 


2 269 


2 048 


113.4 


102.3 


15.87 


2 011 


1800 


1624 


71.33 


64.36 


20.01 


1595 


1427 


1288 


44.86 


40.48 


25.23 


1265 


1 132 


1021 


28.21 


25.46 


31.82 


1003 


897.6 


809.9 


17.74 


16.01 


40.12 


795.3 


711.8 


642.3 


11.16 


10.07 


50.59 


603.7 


564.5 


509.4 


7.017 


6.332 


63.79 


500.1 


447.7 


404.0 


4.413 


3.982 


80.44 


396.6 


355.0 


320.3 


2.776 


2.504 


101.4 


314.5 


281.5 


254.0 


1.746 


1.575 


127.9 


249.4 


223.3 


201.5 


- 1.098 


0.990 6 


161.3 


197.8 


177.1 


159.8 


0.690 4 


0.623 


203.4 


156.9 


140.4 


126.7 


0.434 2 


0.3918 


256.5 


124.4 


111.4 


100.5 


0.273 1 


0.246 4 


323.4 


98.66 


88.31 


98.68 


0.1717 


0.155 


407.8 


78.24 


70.03 


63.19 


0.108 


0.097 46 


514.2 


62.05 


55. .54 


50.11 


0.067 93 


0.061 29 


648.4 


49.21 


44.04 


39.74 


0.042 72 


0.038 55 


817.6 


39.02 


34.93 


31.52 


0.028 87 


0.024 24 


1031 


30.95 


27.70 


24.99 


0.016 90 


0.015 25 


1300 


24.54 


21.97 


19.82 


0.010 63 


0.009 588 


1639 


19.46 


17.42 


15.72 


0.006 683 


0.006 030 


067 


15.43 


13.82 


12.47 


0.004 203 


0.003 792 


2 607 


12.24 


10.96 


9.886 


0.002 643 


0.002 385 


3 287 


9.707 


8.688 


7.840 


.0.001 662 


0.001500 


4 145 


7.698 


6.890 


6.217 


0.001045 


0.000 943 6 


5 227 


6.105 


5.464 


4.930 


0.000 657 5 


000 .593 3 


6 591 


.4.841 


4.333 


3.910 


0.000 413 5 


0.000 373 1 


8 311 


3.839 


3.436 


3.101 


0.000 260 1 


0.000 234 7 


10 480 


3 045 


2.725 


2.459 


0.000 163 6 


0.000 147 6 


13 210 


2.414 


2.161 


1.950 


0.000 102 9 


0.000 092 81 


16 660 


1.915 


1.714 


1.547 


0.000 064 54 


0.000 058 24 


21010 


1.519 


1.359 


1.226 


0.000 040 68 


0.000 036 71 


26 500 


1.204 


1.078 


0.972 6 


0.000 025.^9 


000 023 09 


33 410 


955 


0.854 8 


0.771 3 



404 



TABLE F 



« COPPER WIRE TABLE OF AMERICAN 
Giving Resistances of cool, warm and hot wires, 



B. & S. 




Ohms per Pound. 




or 








A. W. G. 


68° F. 


122° F. 


176° F. 




20° C. 


50° C. 


80° C. 


0000 


0.000 076 39 


0.000 085 35 


0.000 094 59 


000 


0.000 1215 


0.000 135 7 


0.000 150 4 


00 


0.000 193 1 


0.000 215 8 


0.000 239 1 





0.000 307 1 


0.000 343 1 


0.000 380 3 


1 


0.000 488 3 


0.000 545 6 


0.000 604 


2 


0.000 776 5 


0.000 867 5 


0.000 9614 


3 


0.001 235 


0.001 379 


0.001 529 


4 


0.001963 


0.002 193 


0.002 431 


5 


0.003 122 


0.003 487 


0.003 865 


6 


0.004 963 


0.005 545 


0.006 145 


7 


0.007 892 


0.008 817 


0.009 772 


8 


0.012 55 


0.014 02 


0.015 54 


9 


0.019 95 


0.022 29 


0.024 71 


10 


0.031 73 


0.034 5 


0.039 28 


11 


0.050 45 


0.056 36 


0.062 46 


12 


0.080 22 


0.089 62 


0.099 32 


13 


0.127 6 


0.142 5 


0.157 9 


14 


0.202 8 


0.226 6 


0.251 1 


15 


0.322 5 


0.360 3 


0.399 3 


16 


0.512 8 


0.572 9 


0.634 9 


17 


0.815 3 


0.910 9 


1.010 


18 


1.296 


1.448 


1.605 


19 


2.061 


2.303 


2.552 


20 


3.278 


3.662 


4.058 


21 


5.212 


5.823 


6.453 


22 


8.287 


9.259 


10.26 


23 


13.18 


14.72 


16.32 


24 


20.95 


23.41 


25.94 


25 


33.32 


37.22 


41.25 


26 


52.97 


59.18 


65.59 


27 


84.23 


94.11 


104.3 


28 


133.9 


149.6 


165.8 


29 


213.0 


237.9 


263.7 


30 


338.6 


378.3 


419.3 


31 


538.4 


601.6 


666.7 


32 


856.2 


956.5 


1060 


33 


1361 


1521 


1685 


34 


2 165 


2 418 


2680 


35 


3 441 


3 845 


4 262 


36 


5 473 


6 114 


6 776 


37 


8 702 


9 722 


10 770 


38 


13 870 


15 490 


17 170 


39 


22 000 


24 580 


27 240 


40 


34 980 


39 080 


43 320 



TABLE F 



405 



INSTITUTE OF ELECTRICAL ENGINEERS 
of Matthiessen's standard of conductivity. 



Ohms per foot. 


B. &S. 








or 


68° F. 


122° F. 


176° F. 


A. W. G. 


20° C. 


50° C. 


80° C. 




0.000 048 93 


0.000 054 67 


0.000 060 58 


0000 


0.000 06170 


0.000 068 93 


0.000 076 40 


000 


0.000 077 80 


0.000 086 92 


0.000 096 33 


00 


0.000 098 11 


0.000 109 6 


0.000 121 5 





0.000 123 7 


0.000 138 2 


0.000 153 2 


1 


0.000 156 


0.000 174 3 


0.000 193 2 


2 


0.000 196 7 


0.000 219 8 


0.000 243 5 


3 


0.000 248 


0.000 277 1 


0.000 307 1 


4 


0.000 312 8 


0.000 349 5 


0.000 387 3 


5 


0.000 394 4 


0.000 440 6 


0.000 488 3 


6 


0.000 497 3 


0.000 555 6 


0.000 615 8 


7 


0.000 627 1 


0.000 700 7 


0.000 776 5 


8 


0.000 790 8 


0.000 883 5 


0.000 979 1 


9 


0.000 997 2 


0.001 114 


0.001 235 


10 


0.001257 


0.001 405 


0.001557 


11 


001586 


001771 


0.001963 


12 


0.001999 


0.002 234 


0.002 476 


13 


0.002 521 


0.002 817 


0.003 122 


14 


0.003 179 


0.003 552 


0.003 936 


15 


0.004 009 


0.004 479 


0.004 964 


16 


0.005 055 


0.005 648 


0.006 259 


17 


0.006 374 


0.007 122 


0.007 892 


18 


0.008 038 


0.008 980 


0.009 952 


19 


0.010 14 


0.01132 


0.012 55 


20 


0.012 78 


0.014 28 


0.015 83 


21 


0.016 12 


0.018 01 


0.019 96 


22 


0.020 32 


0.022 71 


0.025 16 


23 


0.025 63 


0.028 63 


0.03173 


24 


0.032 31 


0.036 10 


0.041 10 


25 


0.040 75 


0.045 52 


0.050 45 


26 


0.05138 


0.057 40 


0.063 62 


27 


0.064 79 


0.072 39 


0.080 22 


28 


0.081 70 


0.09128 


0.1012 


29 


0.103 


0.115 1 


0.127 6 


30 


0.129 9 


0.145 1 


0.160 8 


31 


0.163 8 


0.183 


0.202 8 


32 


0.206 6 


0.230 8 


0.255 8 


33 


0.260 5 


0.2910 


0.322 5 


34 


0.328 4 


0.366 9 


0.406 7 


35 


0.414 2 


0.462 7 


0.512 9 


36 


0.522 2 


0.583 5 


0.646 6 


37 


0.658 5 


0.735 7 


0.815 4 


38 


0.830 4 


0.927 7 


1.028 


39 


1.047 


1.170 


1.296 


40 



406 



TABLE G 



T.\BLE OF CARRYING CAPACITY OF WIRE AS ESTABLISHED BY THE 
NATIONAL BOARD OF UNDERWRITERS 







Rubber Cover- 


Other 


Copper 


Circular 


ed Wires, 


Insulations 


B. & S. G. 


Mils. 


Amperes 


Amperes 


18 


1 624 


3 


5 


16 


2 583 


6 


8 


14 


4 107 


12 


16 


12 


6 530 


17 


23 


10 


10 380 


24 


32 


8 


16 510 


33 


46 


6 


26 250 


46 


65 


5 


33 100 


54 


. 77 


4 


41 740 


65 


92 


3 


52 630 


76 


110 


2 


66 370 


90 


131 


1 


83 690 


107 


156 





105 500 


127 


185 


00 


133 100 


150 


220 


000 


167 800 


.177 


265 


000 


211 600 


210 


312 




200 000 


200 


300 ^ 




300 000 


270 


400 




400 000 


330 


500 




500 000 


390 


590 




600 000 


450 


680 




700 000 


500 


760 




800 000 


550 


840 




900 000 


600 


920 




1 000 000 


650 


1000 




1 100 000 


690 


1080 




1 200 000 


730 


1 150 




1 300 000 


770 


1220 




1 400 000 


810 


1 290 




1 500 000 


850 


1 360 




1 600 000 


890 


1 430 




1 700 000 


930 


1490 




1 800 000 


970 


1 550 




1 900 000 


1 010 


1 610 




2 000 000 


1 050 


1 670 



The question of drop is not taken into consideration in the 
above table. 



TABLE H 



407 



DIAMETER OF WIRE WHICH WILL FUSE WITH GIVEN CURRENT 
(W. H. Preece.) 





Diameter in Mils 


Amp. 


Copper 


Alum- 


Plat- 


German 


Plat- 


Iron 


Tin 


Tin lead 


Lead 






inium 


inium 


silver 


inoid 






alloy 




1 


2.1 


2.6 


3.3 


3.3 


3.5 


4.7 


7.2 


8.3 


8.1 


2 


3.4 


4.1 


5.3 


5.3 


5.6 


7.4 


11.3 


13.2 


12.8 


3 


4.4 


5.4 


7.0 


6.9 


7.4 


9.7 


14.9 


17.3 


16.8 


4 


5.3 


6.5 


8.4 


8.4 


8.9 


11.7 


18.1 


21.0 


20.3 


5 


6.2 


7.6 


9.8 


9.7 


10.4 


13.6 


21.0 


24.3 


23.6 


10 


9.8 


12.0 


15.5 


15.4 


16.4 


21.6 


33.4 


38.6 


37.5 


15 


12.9 


15.8 


20.3 


20.2 


21.5 


28.3 


43.7 


50.6 


49.1 


20 


15.6 


19.1 


24.6 


24.5 


26.1 


34.3 


52.9 


61.3 


59.5 


25 


18.1 


22.2 


28^6 


28.4 


30.3 


39.8 


61.4 


71.1 


69.0 


30 


20.5 


25.0 


32.3 


32.0 


34.2 


45.0 


69.4 


80.3 


77.9 


35 


22.7 


27.7 


35.8 


35.6 


37.9 


49.8 


76.9 


89.0 


86.4 


40 


24.8 


30.3 


39.1 


38.8 


41.4 


54.5 


84.0 


97.3 


94.4 


45 


26.8 


32.8 


42.3 


42.0 


44.8 


58.9 


90.9 


105.2 


102.1 


50 


28.8 


35.2 


45.4 


45.0 


48.0 


63.2 


97.5 


112.9 


109.5 


60 


32.5 


39.7 


51.3 . 


50.9 


54.2 


71.4 


110.1 


127.5 


123.7 


70 


36.0 


44.0 


56.8 


56.4 


60.1 


79.1 


122.0 


141.3 


137.1 


80 


39.4 


48.1 


62.1 


61.6 


65.7 


86.4 


133.4 


154.4 


149.9 


90 


42.6 


52.0 


67.2 


66.7 


71.1 


93.5 


144.3 


167.1 


162.1 


100 


45.7 


55.8 


72.0 


71.5 


.76.2 


100.3 


154.8 


179.2 


173.9 


120 


51.6 


63.0 


81.4 


80.8 


86.1 


113.3 


174.8 


202.4 


196.4 


140 


57.2 


69.8 


90.2 


89.5 


95.4 


125.5 


193.7 


224.3 


217.6 


160 


62.5 


76.3 


98.6 


97.8 


104.3 


137.2 


211.8 


245.2 


237.9 


180 


67.6 


82.6 


103.6 


105.8 


112.8 


148.4 


229.1 


265.2 


257.3 


200 


72.5 


88.6 


114.4 


113.5 


121.0 


159.2 


245.7 


284.5 


276.0 


225 


78.4 


95.8 


123.7 


122.8 


130.9 


172.2 


265.8 


307.7 


298.6 


250 


84.1 


102.8 


132.7 


131.7 


140.4 


184.8 


285.1 


330.1 


320.3 


275 


89.7 


109.5 


141.4 


140.4 


149.7 


196.9 


303.8 


351.8 


341.3 


300 


95.0 


116.1 


149.8 


148.7 


158.6 


208.6 


322.0 


372.8 


361.7 



408 



* TABLE I 

RING ARMATURE WINDINGS 



Suipui^Vl 
JO sadAx j 


1-H 




(M 


CO 


-^ 


»o 




CO 






u 






;-i 




^1^ 


a 


O'^ o 


a 


a. 


073 <D 


Oh 


•1- 


^^O^ 


•1- 


•1- 


^'B3 


1 


« 0) ^ 


o 


.1. ~f-> 


o 


o 


.1. -t^ 







O 

CO 


CO <^ « 

00 


o 

CD 
CO 


CO 


b^ 

CO "^ fi 
CO 


CO 


1 




cq A 






C^ ^ 






r-\ 


ai 


CIh 


Ph 


.sd 


Oh 


6 




^^ 






^^ 




uctors 
Series 
bween 
ushes 


.1. 


.1. 


.1. 


Oh 






1 


•j* 


i' 


.]• 




1 


a.SJm 


csi 


s; 


S3 


CS3 


S3 


•r 

s: 


OCL, 






Oi 




X 

bC 

-H 


1 


X Oh 

4^ K^ 




CSI 




(N 




(M 


W^ 


O^ 












(M 














^ 




be 


tS3 


ail 

Tl CD 


s: 


<M 


bC 


'A 
S3 


Oh 


X 


Oh 

X 


tfS 




C^ ' ^^ 


(M 1 




(Nl 


S3 




^ m = 














<i^S PPI J 


o 


tH 


1— 1 


o 


^ 


'"' 


JS3 


W 


(N |s^ 


^ 


bC 


d [ 


4 


i " 


bC 


4^ 

f 




-H 


(M 


^ 


41 








t-2 


4 


sa 


^ 




4 


bC 


<N 


-fl 
fl 


0^ 


T3 








'i 


1 


bC 




No. Cir- 
cuits in 
Parallel 
through 
Arm. = b 














a 


(N 


-";: 3 o) p 

^fl > C i- 


a 
X 


X 


^ 
>< 


1 






rfi rt C/^ a; 








;-i 


1 








tH £h ' ,' C<1 


i.s^g' 










-5 =3 oj c 


'^ :fl'^''^^ 


a 


(N 


XI 


Oh 


C^ 


QOoflgi^ 














-0 > O) i- 


Ph 












Tt< - ^^ C 


. ' 1 








(h ^ 






o. Sepa- 
teWind 
gs= X 


tH 


tH 


1— • 


° ^ CD ^ 





1 


>: ^ a 












<^u-^ , 


I 




(M 33 


' 




auipuiAv 


rH 1 CSI 


^ 


Tt< 


* 


CO 


JO fiadAx 








1 




1 










\ 



to 

o 

"B, 

3 

O S-3 
ii 3 e» 

"5 cS s ■ 



3 3 3 



S 0.0.Q, 
w 3 3 3 

d ... 



I 

1^ 

>» 

3 

. o 

Jl 

a ^ M 

S^a 

oj a §« 

fci 3 e3 

111 



a 



r-Cf^CO . 



* TABLE J 

WAVE-WOUND DRUM ARMATURE WINDINGS 



409 



Suipui^ 
JO sadAx 


1 


r-i 






Cs 




CO 




-^ 


d 

i 
ll 


360°-^por 
any odd 
multiple 


1 
-6 


o 

•4-3 


1 

=3 


c 




S 


5 


1 

^ 


Inductors 
in Series 


•1- 

tS3 




•1- 


X 

•I- 

SI 


! 

3 ll 

S.I 

5^ 




Q. 


41 


A 

« 




a. 




a. 
X 






P 

Must be odd 
and have no 

common 
factor with Z 


X 


^.2 a+ " 


-H A 


IS3 


^ 


xu = 


1-H 


1—1 


tH 


T-i 


'o ii 

ll 


-H 


-H 


'i 


X 

i 

X 




No. Cir- 
cuits in 
Parallel 
through 
Arm. = b 


ca 


^ . 


4, 6, 8, or 
any other 
even num- 
ber great- 
er than 2 


X 




No. Cir- 
cuits in 
Parallel per 
Winding 
= bi 


N 


N 


4, 6, 8, or 
any other 
even num- 
ber great- 
er than 2 


2, 4, 6, or 

any other 

even 

number 


No. Sepa- 
rate Wind- 
ings = X 


r-i 


2, 3, 4, or 

any other 

whole 

number 


tH 


o fe 
<N 3 


ga 


SuTpujAV 1 




rH 






c^ 


1 


CO 




•<*< 





i 



3 



d 



|-^ 
^^ 

^ .- 

§§• 

it 

•-.3 
^^ 



^ 



II 
II 



410 



* TABLE K 



LAP-WOUND DRUM ARMATURE WINDINGS 



1 ^"IPuiAV 
1 JO sad^x 


1 - 


(M 


CO 


1 ^ 

1 


ill 


1 Ph 
o'* 
CO 


•1- 
c 

§ 

CO 


•1- 

1 

CO 


o 
CO 

CO 


oil 


a 


&. 


a* 


A 


•oe'S 


•I- 

CS3 


•1- 

ISI 


Si 

•1- 


•I- 


I' 

^ II 


T 


1 
>5" 


1 


1 


|!l 


= 2 or less 
than Z^ p 
must be 
odd 


1 


1 


1 




tH 








Result- 
ant Pitch 

= y 


-H 


X 

-fl 


^ ■ 
-H 


-H 


in = 
d^^S PPM 


o 


O 


o 


o 


Number 

of In- 
ductors 
= Z 


li 


II 


O 

II 


II 


No, Cir- 
cuits in 
Parallel 
through 
Arm. =b 


P. 


p. 


4, 6; 8, or 

any other 

even 

number 

greater 

than 2 


X 


No. Cir- 
cuits in 
Parallel per 
Winding 
= bi 


a 


Ph 


4, 6, 8, or 

any other 

even 

number 

greater 

than 2 


X 

1 


No. Sepa- 
rate Wind- 
ings = X 


rH 


2, 3, 4, or 

any other 

whole 

number 


y-4 


2, 3, 4, or 

any other 

whole 

number 


Buipui^ 
JO sadAx 


r-* 


c 1 


CO 


T}^ 



TABLiii M 



411 



WIRING TABLE FOR TWO PER CENT LOSS ON A 50-VOLT CIRCUIT 
Table for 1 Volt Loss 



O 












CO(MOC5t^COcOiO Tt< rt« CO CO <M T-4 1- 



■^(Mf-<05Q0Ir^cOiO lO Tfi -^ CO <M <M T-i T-H O O O _ O O 
i-t »-i 1— t oo>^oo 

goo 
oo 



Tt<C0T-i<O00I>.t^<:O iO»OTj<rJ<CO<NiM'-<0000000 

^ ^ ^ ^ ooooo 



if 



CO <M O 05 OO t>. t^ cOtO^O"^-^CO(M(Mi— lOOOOOOOO 

'"' wOOO 



Tt< CO 1— I O 05 oo t>» !>• <;0 CO »0 Tt< -^ CO CO C<l r-H o O 



»0 ^ <N tH O 0> oo 00l>.l>.e040»0'*rt<C0<N^'>^OOOOOOOOOC 



CO |Tt< CO t-t O O OS OOOOt^l>»cO»OiOrtiCOCO<MT-iT-ioOO 



COfcTj* CO t-t O O 



3j2 



«D|Tt< CO <N ^ O O 



OSO500l>.t>-cOCO »Ort< COCO (M(MT-(T-t ,—10 00 



88 



CO loaco (M (M ^ oo 

•rH T-H It-H t-I t-H t-H t-I ,-H 



CO lO |CO (M (M '-H 0005aiOOI>-l:^«OiOiO'^COCO(NC<l<M»-<T-i»-iOOC 



»0 BtJ* CO (M T-t ^-HOOOSOOOOO-I>.COif5Tt<TtlCOeOCO(M(M'-*i-ii-iOO 



CO|tH CO CO (M r-li-tOO050000t>.C0C0»0"«i<Tt<C0C0C0Ca<M(Mt-H,-tO 



cO|tH 



CO lO |Tj« CO <M <Mi-li-lOO0>0000t^cOC0iOTti-^'<^C0C0<M<M(M»-HT-t 



CO iO|Tj« CO <M (M^i-HOO 



CO CO cvi (>Ji ,-1 o o 05 OS 00 !>• CO CO »o »o -rt< Tji xfi CO CO CO (M cq 



I »OfTt< CO COCVIC^l^O 



CO CO lO iTt* COCOfMC^^^OOOSOOOOI^-COCOUaiOO-Tj^Tt^-^COCOfM 



CO CO lOlTt* COCO(M<M^OO 



CO CO »0 ■'^ rl<COCO(MT-Hi— i0050SOCI>-t^COCOCO^C»0»0'^'*eO 



• CO CO »0 i-^ rJ4 CO CO (M T-H 1-H O 

• T-H 1— t T— I It-H 1— I t— I 1— I ,— < 1— I t-I t— I 



^^•c^co^ujor^ «>'=^ss;2:5S2gc5^j$?§j*Si§ssgt28g8 



412 



TABLE N 



WIRING TABLE FOR TWO PER CENT LOSS ON A 110-VOLT CIRCUI' 
Table for 2.2 Volts Loss 



I 

IT 



52; 

.0 



o s 

^ 2 






8 



<Ni-HOJ00t^«OiO»O»O-^CO(MC^^ 



1-HOOOOOO 

00000 
0000 



'Tt< CN T-H OS 00 t-» t^ iOiTi iO '^COC<J (MrHOOOO 



sss 



io|cO (M O 



Oi CO t>- «0<0 toiO -^COCO <M(M»-HOOOOOOC 

OOOOOC 

OOOC 



iOh-^ c^ T-H C5 00 00 l>.t^ co»o m^CO COC^i— I,— 10 



88^ 



rji CO ^ O a> 00 I>.t^ t-'CO iOiOTt< 



Tj4 CO cq T-i r^ 00 00000 
000000 



^1 



10 -rt* <M »-< O Oi GOOO t^t^ <OiO«0 '^TJHCOCM^-H.— lOOC 



10 Irfi (M 1-1 O 05 OiOC OOt^ l>.cOiO »C'^eOCO(Mr-"'-<0000000 
'"' '^ '"' 080S 

00 



ogco ^ ^H 



O OiOi 0000 I>.«0«0 10 10 -rt* CO CM <M »-< T-H O 



ssiii 



10 1-^ <M '-I 1-H 005 0500 OOt^l^ COiOTt^-^COeMCM^Ht-iOOOOC 

-^ -^ ^ ^ ^ =^^8c 



DM 



to|^ CO CM »-• i-HO 0S05 Qor>.t^ t^ !£> lo -^j* •<*< CO CO CM »-H 1-4 o o o 



10 1'^ CO CM i-H 1— t O O 



O5CXJ00 t^ C^ «D lO r*4 Tt< CO CO CM 



^^000 



iO|^ I 



q,— I i—iO OiOSCX) 00 !>• «0 «0 »0 -^ -^ CO CO CM '-I »-• O O 



<£> lOi-^ CO CMCM T-i 1— I O 



CVl T-i 1— I O0S05 00 CX5 1^ «0 iO 10 "Tt^-^COCMCMi-Hi— (O 



JOirt* ''J^ COCM CM^ ^OOi OJ00t^I>»'^«:>^'^"^C*5C^CM'-'^ 
?0 ^1^ rf^CO CMCM ;:J^:jO OiOiCCt^t^^^^^^COCOC^'^ 



CO ^n M^ ^ coco cM'-Hr-i 00 



,_,,_! 00 asoot^t^«ocD»0'<t"Tj<cococM 



CO »o>o||^w< c^32S2 ^1^ ;:;J 2 '^ °° ** ^ '^ ^ ^ "^ '* '^ "^ 

coco iCiOi^'^CO CM Cq"^ O O OJ Of 00 t^ t>- <X5 CO >0 rt< 



■^COCMi-t'-<OOOiOOOOt^t^CO 



• CO cOiO^t^-^eocMi-t^OO 



^ ^- ^ CO ^ « CO t^oo oso cm;2J^ S^c5^w§^SSSS88| 



TABLE 



IRING TABLE FOR TWO PER CENT LOSS ON A 220-VOLT CIRCUIT 
Table for 4.4 Volts Loss 









DISTANCE IN FEET TO CENTER OF DISTRIBUTION 






(Wire sizes in B. & S. Gauge) 








No. of 


o 


o 


o 


o 


o 


o 


o 


o 


o 


o 


^ 


o 


o 




o 




o 










CO 








i>. 




















































— 


<M 


«M 


CO 


CO 


■^ 


1 










16 


1.5 
























16 


15 


15 


16 


15 


16 
15 


15 


15 


2 


14 
12 


i— JT 


3 


ir 


"TT 


~T3 


1 12 


4 




















16 


15 


15 


n 


g 


13 


12 


12 


11 


11 


5 


















16 


15 


14 


Ti 


13 


13 


12 


11 


11 


10 


10 


6 














16 


15 


15 


n 


14 


13 


12 


12 


11 


11 


10 


9 


9 


7 












16 


15 


!4 


14 


14 


13 


12 


12 


111 


11 


10 


9 


9 


8 


8 










16 


15 


15 


14 


14 


13 


12 


12 


11 


11 


10 


9 


9 


8 


8 


9 










15 


15 


14 


14 


13 


12 


12 


11 


11 


10 


9 


9 


8 


8 


7 


10 








16 


15 


IS 


14 


13 


13 


12 


11 


11 


10 


10 


9 


8 


8 


7 


7 


12 






16 


15 


14 


14 


13 


12 


12 


11 


11 


10 


9 


9 


8 


8 


7 


7 


6 


14 




16 


15 


14 


14 


13 


12 


12 


11 


11 


10 


9 


9 


8 


7 


7 


6 


6 


5 


16 




16 


15 


14 


13 


12 


12 


11 


11 


10 


9 


9 


8 


8 


7 


7 


6 


6 


5 


18 




15 


14 


13 


12 


12 


11 


11 


10 


9 


9 


8 


8 


7 


7 


6 


5 


5 


4 


20 


16 


15 


14 


13 


12 


11 


11 


10 


10 


9 


8 


8 


7 


7 


6 


5 


5 


4 


4 


25 


16 


n 


13 


12 


11 


10 


10 


9 


9 


8 


7 


7 


6 


6 


5 


4 


4 


3 


3 


30 


15 


13 


12 


11 


10 


10 


9 


9 


8 


7 


7 


6 


6 


5 


4 


4 


3 


3 


2 


35 


7? 


13 


11 


10 


10 


9 


8 


8 


7 


7 


6 


5 


5 


4 


4 


3 


2 


2 


1 


40 


14 


12 


11 


10 


9 


8 


8 


7 


7 


6 


5 





4 


4 


3 


2 


2, 


1 


1 


45 


13 


12 


10 


9 


9 


8 


7 


7 


6 


6 


5 


4 


4 


3 


3 


2 


1 


1 





50 


13 


11 


10 


9 


8 


7 


7 


6 


6 


5 


4 


4 


3 


3 


2 


1 


1 








60 


12 


10 


9 


8 


7 


7 


6 


6 


K 


4 


4 


3 


3 


2 


1 


1 








00 


70 


11 


10 


8 


7 


7 


6 


5 


5 


4 


4 


3 


2 


2 


1 


1 





00 


00 


000 


80 


11 


9 


8 


7 


6 


5 


5 


4 


4 


3 


2 


2 


1 


1 





00 


00 


000 


000 


90 


10 


9 


7 


6 


6 


5 


4 


4 


3 


3 


2 


1 


1 





00 


00 


000 


000 


000 


100 


10 


8 


7 


6 


5 


4 


4 


3 


3 


2 


1 


1 








00 


000 


000 


000 


000 


120 


9 


7 


6 


5 


4 


4 


3 


3 


2 


1 


1 








00 


000 


000 


000 I 


000 





414 



SYMBOLS FOE WIRING PLANS 



■ 



STANDARD SYMBOLS FOR WIRING PLANS 

A« ADOPTKU ANI> KKCOMMKNUED «T 

m lUTlOMl ELECTUCAl CftimtAaORS ASSOCIATION OF THE UMITED STATES and TK AMERICAN INSTITUTE OF AlCIITtCn. 



i£l. Ceiling Outlet; ^ectiio oal;. Hwaertl in center iadicatet aomDcr of Standard 16 C. P. Inoa&detcent Laapt. 
jS f Ceiling Ontleti-CoabiaAtion. { indioatet i-16 0. f. Standard Incandescent Lamp* and 3 Oas Bomen. U e&* only jM 

^^^ Bracket Outlet; Electric only. Hnmeial in center indicatei number of Standard 16 C. ?. IneandcKent Lamps. 
^tim^'i Bracket Outlet; Combination, j indicates 4-16 C P. Standard Incandescent Lamps and 2 Oas Burners. If gas only ^-jV 
^~|1] Wall or Baseboard Beceptade Outlet. Humeral in center indicates number of Standard 16 C. P. Incandescent La mpa. 
^^ floor Ontlet. Humeral in center indicates number of Standard 16 C. P Incandescent Lamps. 
^6 Outlet (or Outdoor Standard or Pedestal; Electric only. Humeral indicates number of Stand. 16 C. P. Lamps. 
]^f Outlet for Outdoor Standard or Pedeital; Combination. | indicates 646 C. P. Stand. Incan. Lampa; 6 Oa* Baist<« 
^ Drop Cord Outlet 
One Light Outlet, for Lamp BeecjtMl*. 
(^ Arc Lamp Ontlet <^ 

^ Special Outlet for lightiBK, Hwtisf tad Power Current, u described in Speciflcationi. 
^Cy^ Ceiling Fan Outlet. 

\ Show u many Symbols as there are Switches. Or in case of a very large group 
of Switches, indicate number of Switches by a Boman numeral, thus: S> ZII, 
meaning 12 Single Pole Switches. 
Describe Type of Switch in Specifications, that is, 
\ Plush or Surface, Push Button or Snap, 



S' 
5' 



B 



8. P. Switch Outlet. 
D. P. Switch Outlet. 
3-Way Switch Ontlet. 
4- Way Switch Outlet 
Automatic Boor Switch Outlet 
Electrpli<r Switch Outlet 
Meter OuUet. 

I Bistribution Panel 

Rnnm Junction or Pull Bex. 

J^ Motor Ontlet; Humeral in ototer indicatet Hone Power. 
fx3 Motor Control Ontlet 
^f~te Transformer. 
■■»— > III Main or Feeder run concealed under Floor, 

^■i^^-^i— ^ Main or Feeder run concealed under Floor abors. 
——— ——— Main or Feeder run exposed. 

^— — — — Branch Circuit mn concealed nnder Floor. 

•— — — — Branch Circuit mn concealed nnder Floor abon. 



- Branch Cironlt mn expoeed. 

- Pole Line. 



SUCCESTIONS IN COIVNECTION WITH STAN- 
DARD SYMBOLS FOR WIRINC PUNS 

It is important that ample space be 
allowed for the installation of mains, feed- 
ers, branches and distribution panels. 

It is desira'ble that a key to the symbols 
used accompany all plans 

If mains, feeders, branches and dis- 
tribution panels are shown on the plans, 
it is desirable that they be designated by 
letters or numbent. 
Heights of Centre of Wall Outlets (uolcss 

otherwise specified) 

Living Rooms 8' 6" 

Chambers 6' 0" 



Offia 



Corridors 
Height of Switches (unless otherwise spec- 
ified) - 4' 0" 



^ Telephone Outlet; Wivate Service. 

y Telephone Outlet; Public Service. 

g Bell OuUet 

Q Butrer Ontlet 

Q2 Push Button Outlet ; Humeral indicates number of Puihet. 

--^^ Annunciator; Humeral indicates Msfcar W Pointa. 

— ^ Speaking Tube. 

_^ Watchman Clock Outlet. 

I Watchman SUtion Outlet. 

_^ Master Time Clock Ontlet. 

__J|^ Secondary Time Clock Outlet. 

m Door Opener. 

R« Special Ontlet ; Tor Signal Systems, as described In Speeifleatioai. 

j||||| Battery OuUet. 

( Circuit for Clock, Telephone, Bell or other Service, ran under Floor, eoaeealed. 
I Kind of Serriee wanted ascertained by Symbol to which line connects. 
I Circuit for Clock, Telephone, Bell or ether Service, ran nnder Floor above, concealed. 
^^~** \ Xiad ol Service wanted atoertained by Symbol to wluch Une eonnects. 

ROra— If other than Standard 16 C. P. Incandescent lampa are iainS, 
SpMifleations should describe capacity of Lamp to be ased. 



6' 0- 
6' 3- 



Copy righted 



INDEX 

A 

PAGE 

Admittance of a circuit 349 

Alternating-current circuit 332 

addition and subtraction of vectors 338 

alternation 333 

chemical and heating effects of 334 

cycle 333 

definition of alternating current 332 

determining value of power factor.. , 354 

electromotive force required to overcome resistance 339 
electromotive force required to overcome resistance, 

inductance, and capacity 344 

factors determining value of a. c 338 

frequency 333 

hydraulic analogy of alternating! current 332 

hydraulic analogy of capacity 342 

hydraulic analogy of inductance 339 

impedance of a circuit 347 

impedance diagram 348 

impedances in parallel 349 

impedances in series 348 

instantaneous power in 353 

maximum, average and effective values of e.m.f . ... 336 

period 333 

phase 333 

phase relation of current and potential drops in a 

divided circuit 352 

phase relation of current and potential drops in a 

series circuit 351 

phase relation of e.m.f. to overcome inductance, etc. 341 

problems on 356 

reactance of a circuit 347 

415 



416 INDEX 

Alternating-current circuit — continued page 

sine wave e.m.f 335 

synchronism 333 

total e.m.f. required to produce a given alternating 

current 346 

vector representation of alternating e.m.f.'s and 

currents 338 

wattmeter indicates true power in 355 

Alternating current machinery 358 

alternators 358 

' connecting receiving circuits to a three-phase system 364 

induction generator 384 

induction motor 379 

measurement of power in single-phase system 365 

measurement of power in threie-phase system 367 

measurement of power in two-phase system 365 

relation of e.m.f. and current in "Y" and "A" con- 
nected armatures 362 

rotating magnetic field 378 

synchronizing 371 

synchronous converter 385 

synchronous motor 368 

transformer 372 

Alternation, definition of 333 

Alternators 358 

inductor 358 

single-phase 360 

with stationary armatures 358 

with stationary fields 358 

three-phase 361 

Amalgamation 60 

Ammeter 122-132 

Ammeter, calibration of 163 

Ammeter shunts 124 

Ampere, definition of 6-10 

Ampere-hour meters 159 

Ampere-turns, definition of 94 

Angle of lag, definition of 182 

Angle of lead, definition of 182 

Armature coil 227 

Armature inductor 226 



INDEX 417 

PAGE 

Armature reaction 180 

Armature reaction, means of reducing 183 

Armatures 219 

armature core 219 

armature-core stampings 220 

ventilation 221 

armature windings 224 

armature coil 227 

armature inductor 226 

drum windings 230 

element of 227 

equipotential connections 235 

front and back pitch, choice of 234 

multiplex windings 232 

number of commutator segments 227 

number of paths through 233 

pitch of winding and field step , 228 

re-entrancy 232 

ring windings » 229 

table 335 

brushes and brush holders 223 

commutator 222 

commutator risers 222 

construction of 223 

Anode, definition of '. 126 

Artificial magnets 77 

Automobile motors 212 

Auto-transformer 377 

B 

Balanced load 261 

Bichromate cell 65 

Bonding, definition of ; 129 

Boosters 267 

Bremer flaming-arc lamp 304 

British thermal heat unit, definition of 142 

Brushes and brush holders 223 

Bunsen photometer. 308 

Bus-bars , 270 



418 INDEX 

C 

PAGE 

Calculation of illumination 310 

Calculation of resistance of conductor 318 

Calculation of size of conductor when allowable drop ' 

and current are given 319 

Calibration of instruments 161 

ammeters 163 

voltmeters 161 

wattmeter 163 

Carbon arc lamp 294 

Carbon-filament lamp 299 

Cathion, definition of 126 

Cathode, definition of 126 

Chemical depolarization 65 

Fuller cell 66 

Grenet cell 65 

Leclanche cell 67 

Chemicals used in cells and their symbols 65 

Choice of material to use as conductor 317 

Circuit breakers 273 

Circular mil, definition of 28 

Closed-coil windings 225 

Commercial wheatstone bridge 54 

Commutation of generator 185 

Commutator pitch 229 

Compensated wattmeter ; 155 

Compound generators in parallel 271 

Compound motor 194 

Compound motor, characteristics of. 208 

Compound-wound generator 179 

Concealed *'knob and tube" work 325 

Condenser 145 

capacity of 145 

connection of in series and parallel 147 

dielectric 145 

problems on 149 

relation of impressed voltage, quantity and con- 
denser capacity 148 

Conductance of a circuit 349 

Conductance of a conductor 18 



INDEX 419 

PAGE 

Conductors 6 

Conductors, area of circular 20 

Constant-current distribution 258 

Constant-voltage distribution ' 257 

Coulomb, definition of 6-9 

Counter electromotive force 199 

Cumulative compound motor 195 

Current, definition of 4 

Current of electricity 6 

Current, uniformity of in series circuit 35 

Cycle, definition of 333 

D 

Daniell cell 64 

Daniell cells, electro-chemical depolarization of 67 

D'Arsonval ammeters 134 

D'Arsonval galvanometer 133 

D'Arsonval voltmeters 134 

Dead-line release 205 

Diamagnetic, definition of , 82 

Dielectric „ 145 

Differential compound motor 195 

Differential shunt 298 

Direct-current dynamos, diseases of 280 

Direct-current generator 166 

adaptability of 189 

armature of a dynamo 173 

armature reaction 180 

back turns 182 

building up of 187 

capacity of 186 

commercial rating of 191 

commutation of 185 

compound-wound 179 

cross-turns 182 

efficiency of 190 

external characteristics of 187 

losses in ' 189 

magnetic field of a dynamo 173 



420 INDEX 



^ 



Direct-current generator — continued page 

magnetic leakage 176 

multiple-coil armatures 171 

problems on 191 

reducing armature reaction 183 

self-excitation of 177 

separate excitation of 177 

series-wound 17S 

simple alternator 168 

simple direct-current dynamo 170 

simple dynapio 166 

shunt-wound 178 

values of induced e.m.f. in armature winding 176 

Direct-deflection method of measuring resistance 50 

Direct-current motors 193 

adaptability of 208 

• armature reaction in 196 

automobile 212 

characteristics of 207 

combined starting and field-regulating rheostats... 206 

compound 194 

construction of 209 

counter electromotive force 199 

dead-line release 205 

efficiency of 215 

elevator and crane 214 

Fleming's left-hand rule 193 

fundamental principle of 193 

interchangeability of motor and generator 193 

interpole motor 203 

mechanical output of , 198 

normal speed of 199 

overload release 206 

position of brushes on 197 

problems on = 217 

railway 210 

regulating speed of 200 

series 194 

shunt - 194 

speed regulation 206 

starting 204 



INDEX 421 

Direct-current motors — continued page 

starting boxes 205 

torque exerted on 198 

Ward Leonard system of speed control 203 

Diseases of direct-current dynamos 280 

dynamo fails to generate 290 

heating of armature 284 

heating of bearings 285 

heating of field coils 284 

motor runs backward or against the brushes 290 

motor stops 289 

noise 287 

sparking at brushes caused by excessive current in 

armature due to overload 282 

sparking at brushes due to fault of armature 283 

sparking at brushes due to fault of brushes 280 

sparking at brushes due to fault of commutator 

or magnetic field 281 

speed too high 288 

speed too low 289 

suggestions and precautions 292 

Disk windings 224 

Distribution curves 310 

Distribution and operation 257 

boosters 267 

circuit breakers 273 

constant-current distribution 258 

compound generators in parallel 271 

constant-voltage distribution 257 

drop in potential in the neutral wire 261 

dynamotors 265 

Edison three-wire system of distribution 259 

ground detectors 275 

instructions for starting a generator or motor 276 

motor generators, or balancers. 267 

operation of compound motors 273 

operation of generators and motors for combined 

output 269 

operation of series motors in series and parallel 272 

operation of shunt motors in series and parallel 272 

rheostat 274 



422 INDEX 

Distribution and operation — continued page 

series generators connected for combined output. . . . 270 

series-parallel system 'of distribution 259 

shunt generators connected for combined output 269 

starting and stopping compound generators that are 

operating in parallel 278 

switchboard 273 

switches 275 

three-wire generators 263 

Drop in potential in the neutral wire 261 

Drum windings 224 

lap 230 

progressive 231 

retrogressive 231 

wave *. 231 

Dry cells 68 

Dynamic brake 215 

Dynamotor as an equalizer 265 

Dynamotors 265 

E 

Eddy currents 117 

Eddy-current loss 118 

Edison chemical meter 159 

Edison storage battery 247 

care of 250 

chemistry of 249 

Edison three-wire system of distribution 259 

Effect different colored walls have upon general illumi- 
nation 314 

Electric heaters 329 

Electric generators and motors 330 

Electric lamp 294 

Electric lighting 294 

Bremer flaming-arc lamp 304 

Bunsen photometer 308 

calculation of illumination 310 

carbon arc lamp 294 

carbon-filament lamp 299 

distribution curves 310 



INDEX 423 

Electric lighting — continued page 
effect different colored walls have upon general illu- 
mination 315 

electric lamp 294 

flaming arc 304 

glow lamp 299 

hefner lamp 307 

mercury vapor lamp 304 

metalized carbon-filament or gem lamp 300 

Moore tube 300 

multiple arc lamps 296 

Nernst lamp 303 

photometry 307 

regulation of arc lamps on constant-voltage and con- 
stant-current circuit 295 

series arc lamps 297 

shades, reflectors, and diffusers 314 

specific consumption of lamps 309 

tantalum lamp 300 

tungsten lamp 301 

units of illumination 307 

vapor lamp , 303 

Electric wiring 316 

calculation of resistance of conductor 318 

calculation of size of conductor when allowable drop 

and current are given 319 

choice of material to use as conductor 317 

concealed "knob and tube" work 325 

electric generators and motors 330 

electric heaters 329 

electrical inspection 330 

factors determining size of conductors 316 

installing arc lamps and fixtures 329 

interior conduit and armored cable work 326 

location of outlets, switches, and distributing board. 328 

methods of wiring and rules governing same. 322 

motor wiring formula 321 

moulding work 324 

open, or exposed, work 323 

service wires 327 

wiring in general 316 



424 INDEX 

PAGi 

Electrical circuit ll 

compared with hydraulic circuit 5 

conductors 6 

coulomb, ampere, ohm, and volt 9 

current of electricity 6 

derived units 15 

electrical force 11 

electrical power 13 

electrical quantities 6 

electrical work or energy 12 

electromotive force 7 

fundamental units 15 

hydraulic analogy of 2 

insulators 7 

Joule, unit of electrical work 12 

kilowatt 14 

kilowatt-hour 15 

mechanical horse-power 14 

Ohm's Law 8 

resistance 6 

watt-hour 15 

Electrical force, definition of 11 

Electrical inspection 330 

Electrical instruments 121 

ammeter 122 

ammeter shunts 124 

calibration of instruments 161 

galvanometer 122 

measurement of power 151 

operation depending on electro-chemical effect 12^ 

electrolysis 125 

electroplating 127 

electrotyping 128 

polarity indicator 128 

weight voltameter 129 

operation depending on electrostatic effect 145 

electrostatic voltmeter 150 

condenser 145 

operation depending on heating effect 141 

hot-wire instruments 143 



INDEX 425 

Electrical instruments — continued page 

operation depending on magnetic effect. 132 

electro-dynamometer 139 

magnetic vane ammeter or voltmeter 138 

fer plunger type ammeter or voltmeter 137 

i tangent galvanometer 132 

I Thomsen inclined-coil instruments 138 

voltmeter 122 

Electrical power, unit of 13 

Electrical quantities 6 

Electrical units 12-16 

derived units 15 

fundamental units 15 

horse-power 14 

Joule 12 

kilowatt ...* 14 

kilowatt-hour 15 

systems of units 16 

watt 13 

watt-hour 15 

Electrical work or energy 12 

Electricity, definition of 1 

Electro-chemical depolarization 67 

Electrodynamometer 139 

Electrodes, definition of 125 

Electrolysis 125-127 

of copper sulphate 127 

definition of 125 

Electrolyte 241 

Electrolytic action, prevention of 128 

Electrolytic cell, definition of 125 

Electromagnet 98 

Electromagnetic induction 102 

currents induced in coil 105 

currents induced in conductor 102 

eddy currents 117 

induced pressures, rules for 116 

inductance 114 

' Lentz's law 115 

magnitude of induced e.m.f 106 

mutual induction 113 



426 INDEX 

Electromagnetic induction — continued page 

non-inductive circuit 119 

primary coils 109 

secondary coil 109 

self-induction 113 

Electromagnetism 86 

direction of an electromagnetic field 86 

electromagnet 98 

law of traction 101 

magnetic field around conductor carrying current... 86 

magnetomotive force 93 

permeability 92 

problems on 97 

reluctance 95 

rules for determining direction of field 88 

solenoid 89 

strength of magnetic field 89 

toroid 92 

Electromotive force 7 

Electromotive force in a circuit, effective 38 

Electromotive force of a cell, factors determining 62 

Electromotive force and potential difference 7 

Electroplating 127 

Electrostatic voltmeter 150 

Electrotyping 128 

Elevator and crane motors 214 

Equalizer 271 

Evershed ohmmeter, principle of 56 

F 

Factors determining size of conductors 316 

Feeder valve 306 

Field rheostat 178 

Flaming arc 304 

Flashing 300 

Fleming's motor rule 193 

Foot-candle 307 

Frequency, definition of 333 

frequency changer 384 

Friction brake 214 

Fuller cell 64 



1 



INDEX 427 
G 

PAGE 

Galvanometer 122 

Gauss, definition of 94 

Generator panels and feeder panels 273 

Gilbert, definition of 94 

Glow lamp 299 

Grenet cell 63-65 

Ground detectors 275 

H 

Heating effect of a current, commercial applications of. . 142 

Hefner lamp 307 

Hefner unit 307 

Henry, unit of inductance 115 

Horse-power 14 

electrical 14 

mechanical 14 

Horseshoe magnet 85 

Hot-wire instruments 143 

Hydraulic analogy of electrical circuit 2 

Hydraulic circuit compared with electrical 5 

Hydrometer 242 

Hysteresis 98 

definition of 83 

loss 99 

I 

Induced electromotive force « , . c 102 

direction of 107 

magnitude of 106 

rules for determining 109 

in secondary coil 109 

Induced pressures, rules for 116 

Inductor alternators 358 

Inductance 114 

Induction generator 384 

Induction motor 379 

methods of starting 382 



428 INDEX 



I 



Indication motor — continued page 

operation of 381 

rotor 380 

speed regulation of 381 

"squirrel-cage" type 380 

stator 380 

Installing arc lamps and fixtures 329 

Instructions for starting a generator or motor 276 

Insulators 7 

Integrating wattmeters 159 

Interior conduit and armored cable work 326 

Internal resistance 62 

International ohm, definition of 10 

Interpole motor 203 

Ions, definition of 126 



I 



J 

Joule, unit of electrical work and energy 12 

K 

Kicking box 3' 

Kilovolt, definition of I'J 

L 

Lalande cell 64 

Lead storage cells 237 

action of, while charging 238 

action of, while discharging 238 

Leakage coefficient 176 

Leclanche cell 63 

Lentz's law 115 

Location of outlets, switches, and distributing board 328 

Logarithms 390 

characteristic, definition of ' 391 

common system of 390 

definition of 390 

to find a number whose log is given 391 

how to obtain log of a number from table 391 

laws of indices 390 

mantissa, definition of 391 



INDEX 429 
M 

PAGE 

Magnet 77 

artificial 77 

permanent 84 

poles of 78 

currents induced in a conductor by 102 

Magnetic attraction 78 

Magnetic circuit, materials used in construction of 175 

Magnetic field of a dynamo 173 

air gap 174 

armature core 174 

field cores 173 

pole face 174 

pole pieces 174 

yoke 173 

Magnetic field of force 80 

distortion of 81 

making of 81 

Magnetic field, strength of 89 

Magnetic force 79 

Magnetic flux 94 

Magnetic induction 82 

Magnetic leakage 176 

Magnetic lines of force 80 

Magnetic meridian 78 

Magnetic needle 78 

Magnetic repulsion 78 

Magnetism 77 

magnet 77 

magnetic attraction 78 

magnetic field 80 

magnetic force 79 

magnetic lines of force. 80 

magnetic induction 82 

magnetic needle 78 

magnetic repulsion 78 

magnetizable metals 79 

molecular theory of 83 

retention of magnetization 82 

Magnetizable metals 79 



430 INDEX 



t 



PAGE 

Magnetization curve 187 

Magnetization, retention of 82 

Magnetomotive force 93 

Maximum demand meters 160 

Maxwell, definition of. 94 

Mean horizontal candle-power 307 

Mechanical depolarization 65 

Megohm, definition of 10 

Mercury vapor lamp 304 

Metalized carbon-filament or gem lamp 300 

Methods of wiring and rules governing same 322 

Microhm, definition of 10 

Mil-foot resistance 29 

Millivolt, definition of i 10 

Molecular theory of magnetism 83 

Moore tube 306 

Motor-generators, or balancers 267 

Motor wiring formula 321 

Moulding work 324 

Multiple arc lamps 296 

Multipliers 136 

Multiplex windings 232 

Mutual induction 113 

N 

Negative booster 268 

Negative grid 238 

Nernst lamp 303 

Neutral wire 260 

No-load current 373 

Non-lead storage cells 246 

Non-inductive circuit 119 

O 

Ohmmeter 56 

Ohm's law 8 

problems on 10 

Open-coil windings 226 

Open, or exposed, work 323 



INDEX 431 

PAGE 

Operation of compound motors 273 

Operation of generators and motors for combined output. 269 

Operation of series motors in series and parallel 272 

Operation of shunt motors in series and parallel 272 

Overload release 206 

Oversulphation 243 

P 

Panels 273 

Paramagnetic, definition of 82 

Period of an e.m.f., definition of 333 

Permanent magnets, application of 84 

Permeability 92 

Phase, definition of 333 

Photometer 307 

Photometer bench 309 

Photometry .' 307 

Polarity, definition of 78 

Polarity indicator 128 

Polarity of solenoids 91 

Polarization 60 

chemical means of prevention 61 

electro-chemical means of prevention 62 

mechanical means of prevention 61 

Poles of a magnet , 78 

Portable storage batteries 251 

Positive grid 238 

Potential difference, relation of, to resistance 36 

Power, measurement of 151 

Power and energy calculations, problems on 16 

Power factor 354 

Pressure, definition of 4 

Primary batteries 58 

amalgamation 60 

cell requirements 70 

chemical action in battery 59 

chemical depolarization 65 

chemicals used in cells 65 

closed-circuit cell 63 



432 INDEX 



J 



Primary batteries — continued PAi 

double-fluid cell 

dry cells 68 

electro-chemical depolarization , 67 

electromotive force of a cell 62 

grouping of cells, problems on 74 

internal resistance 62 

local action in battery 60 

mechanical depolarization 65 

open-circuit cell 63 

parallel connection of cells 72 

polarization 60 

primary cell 63 

secondary cell 63 

series connections of cells 70 

series and parallel combinations of cells 73 

single-fluid cell 64 

standard cells 69 

voltaic cells 58 

Primary cells, forms of 64 

Prony brake 21- 

R 

Railway motors and their control 210 

Reactance coil 29o 

Re-entrant windings 232 

Regulation of arc lamps on constant-voltage and con- 
stant-current circuit 295 

Relative conductivity of a material 30 

Relative resistance of a material 30 

Reluctance of magnetic circuit 95 

Resistance, calculation of 18 

area of circular conductors 20 

changes with temperature 21 

from dimensions and specific resistance 31 

due to change in temperature 2 

meaning of (K) 21 

mil-foot resistance ^ 

relation to physical dimensions 27 

relation between square and circular-mil measure... 3( 






INDRX 433 

Resistance, calculation of — continued page 

relative conductivity 30 

relative resistance 30 

specific resistance 28 

temperature coefficient 21 

varies inversely as cross-section of conductor 19 

varies directly as the length of conductor 18 

Resistance, definition of 4 

Resistance, measurement of 44 

commercial Wheatstone bridge 54 

by comparison 47 

direct-deflection method 50 

drop in potential method 44 

ohmmeter 56 

series voltmeter method 48 

slide-wire Wheatstone bridge 52 

Resuscitation from apparent death from electric shock. . 386 

Reverse-current 274 

Rheostats 205, 274 

Right-hand screw rule 88 

Ring windings 224 

series-connected wave-wound 230 

spirally-wound 229 



Screen 308 

Self-excitation of generator 177 

Self-induction 113 

Separate excitation of generator 177 

Series arc lamps 297 

Series and divided circuits 34 

effective e.m.f. in a circuit 38 

parallel or multiple grouping 39 

problems on 43 

relation of p.d. to resistance 36 

resistance of 37 

series grouping 34 

series and parallel combinations 42 

uniformity of current 35 

iSeries generators connected for combined output 270 



434 INDEX 



■ 



PAGE 

Series motor , 194 

characteristics of 207 

Series-parallel system of distribution 259 

Series voltmeter method of measuring resistance 48 

Series-wound generator 178 

Service wires 327 

Shades, reflectors, and diffusers 314 

Shunt generators connected for combined output 269 

Shunt motor 194 

characteristics of 207 

Shunt-wound generator 178 

Single-phase alternator 360 

Slide-wire Wheatstone bridge, principle of 52 

Solenoid 89 

permeability 92 

polarity of 91 

toroid , 92 

Specific consumption of lamps 309 

Specific inductive capacity 145 

Speed regulation of a motor 206 

Specific resistance 28 

Square and circular-mil measure, relation between 30 

Standard cells 69 

Starting and stopping compound generators that are op- 
erating in parallel 278 

Storage batteries 237 

to aid generators in carrying maximum load 253 

capacity of 241 

commercial application of 250 

containing vessel and separators 242 

Edison 247 

in electrical laboratories 251 

electrolyte 241 

hydrometer 242 

lead cell 237 

management of 245 

non-lead cell 246 

portable 251 

to put out of service 246 

sulphation 243 



INDEX 435 

Storage batteries — continued page 

to supply energy during certain hours 253 

to supply energy for electrically driven vehicles 

and boats 255 

storage battery grids 239 

storage cell 237 

in telephone and telegraph work 252 

for train lighting 252 

troubles and their remedies 244 

buckling of plates 245 

corrosion of plates 245 

loss of capacity 244 

loss of voltage 245 

shedding of active material 245 

used in changing voltage 254 

used in subdividing voltage of a generator 255 

Storage battery grids 239 

comparison of Plante and Faure 240 

example of 239 

Faure process 239 

Plante process 239 

Sulphation 243 

Susceptance of a circuit 349 

Switchboard 273 

Switches 275 

Synchronism, definition of 333 

Synchronizing 371 

Synchronoscope 372 

Synchronous converter 385 

Synchronous motor 368 

operation of 370 

starting 370 

synchronizing 371 

T 

Table 

approximate specific consumption of different type 

lamps 310 

capacity of storage battery, percentage variation of. 241 
centigrade and Fahrenheit thermometer scales, com- 




436 INDEX 

Table — continued page 

parison of 400 

constants to be used in calculating illumination on 

horizontal plane below lamp 312 

copper wire table of American Institute of Electrical 

Engineers 402-405 

elect! o-chemical equivalents 130 

factors for obtaining effective Illumination 315 

hysteretic constant, value of, for different materials. 100 

lap-wound drum armature windings 410 

logarithms of numbers 394, 395 

mensuration equations 396 

metric and English measures, relation of 399 

primary cells^ 64 

required illumination for various classes of service. 313 

ring armature windings 408 

specific inductive capacities 146 

specific resistance data 29 

symbols for electrical apparatus 398 

symbols for wiring plans 414 

temperature coefficients 22 

temperature coefficient of copper 23 

trigonometrical functions 397 

units in English and metric systems, relation of . . . . 16 

wave- wound drum armature windings 409 

wire, carrying capacity of 406 

wires, equivalent cross-section of different size 400 

wire gauzes 401 

wiring table for 2 per cent loss on a 50-volt circuit. . 411 

wiring table for 2 per cent loss on a 110-volt circuit. 412 

wiring table for 2 per cent loss on a 220-volt circuit. 413 

wire which will fuse with given current, diameter of. 407 

Tangent galvanometer 132 

Tantalum lamp 300 

Temperature coefficient, definition of 21 

Thomson watt-hour meter 158 

Three-phase alternator 361 

Three-wire generators 263 

Toroid 92 

Traction, law of 101 

Transformer 372 



INDEX 437 

Transformer — continued page 

action of, on load , 373 

action of, without load 372 

actual e.m.f. and current relations in 374 

auto-transformer 377 

constant-current 377 

constant-potential 377 

core type 376 

current , 377 

ideal e.m.f. and current relations in 373 

methods of cooling 377 

polyphase 376 

shell type 376 

single-phase 376 

vector diagram of ' 375 

Transposition, definition of 113 

Tungsten lamp 301 

U 

Unbalanced load 261 

Under-load 274 

Units of illumination 307 

V 

Vapor lamp 303 

Voltaic battery 59 

amalgamation 60 

chemical action in 59 

local action 60 

polarization 60 

Voltaic cell , , 58 

Volt, definition of 10 

Voltameter 

adaptability of 131 

definition of 126 

Voltmeter ^ 122 

calibration of 161 



■ 



438 INDEX 

W 

PAGE 

Watt-hour meters 156 

Wattmeter 153 

adaptability of 156 

calibration of 163 

compensated 155 

principle of 153 

Weston 155 

Whitney 154 

Weight voltameter 129 

Weston portable voltmeter 137 

Weston wattmeter 155 

Wheatstone bridge 

commercial 54 

slide-wire 52 

Whitney wattmeter 154 

Wire gauges 32 

Wiring in general 316 

Wright demand meter 160 

Z 

Zero load 372 



THIRD EDITION 

Revised to June 1st, 1911 

VEHICLES OF THE AIR 




BY 



Victor Lougheed 

Member of the Aeronautic Society; 
Formerly Editor of Motor; Founder 
Member Society of Automobile En- 
gineers; Consulting- Engineer of the 
Aero Club of Illinois. 

Absolutely Reliable 

Down-To-The-Minute 

Concise, yet Comprehensive 



A book that has been adopted by foremost Educational Institu- 
tions as a text or reference work, after careful search through- 
out this country and abroad. You will find this book to 
contain a complete list of flights from the earliest to 
the present time; fatalities; also complete working 
drawings (drawn to scale) so that anyone mechan- 
ically inclined could make his own flying machine 
— design, operate and repair. 

PARTIAL LIST OF CHAPTERS 



I. 


The Atmosphere. 


VIII. 


Bearingfs. 


II. 


T.ig:hter-Than-Air Machines. 


IX. 


Lubrication. 


III. 


Heavier-Than-Air Machines. 


X. 


Starting- and Aliqrhtingf. 


IV. 


Aeroplane Details. 


XI. 


Materials and Construction. 


V. 


Propulsion. 


XII. 


Typical Aeroplanes. 


VI. 


Power Plants. 


XIII. 


Accessories. 


vn. 


Transmission Elements. 


XIV. 


Miscellany. 






XV. 


Flight Records. 



A Book of 550 Pages — 500 Subject Headings— 200 Aeronautical 

Terms Defined — 270 Illustrations, Half-tones and Views 

Bound in Cloth, Stamped Front and Back in Gold 

Size 53^ X 9 inches 

Price, $2.50 (net). Postage, 25 cents extra. 



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